2 Variants

2.1 XB-47 / B-47A

The XB-47 was an attractive, " Buck Rogers" aircraft. The 35-degree swept wings were shoulder-mounted, with the twin inboard turbojet engines mounted in a very neat pod, and the outboard engine tacked under the wing short of the wingtips.

The airfoil was 11 times as wide as it was thick. This unusual thinness was required to attain high speed, but the wing's flexibility was a concern. It could flex as much as 1.5 m (5 feet) up or down, and major effort was expended to ensure that flight control could be maintained as the wing moved up and down. As it turned out, most of the worries proved unfounded. The wings were fitted with a set of flaps that extended well behind the wing, or " Fowler flaps ", to assist in takeoffs.

The bicycle landing gear dictated by the thin wing consisted of a pair of large wheels fore and aft of the bomb bay, with small outrigger wheels carried on the inboard twin-jet pods.

The performance of the Model 450 design was projected to be so good that the bomber would be as fast as fighters then on the drawing board, and so the only defensive armament was to be a tail turret with two 12.7 mm Browning machine guns, which would in principle be directed by an automatic fire-control system. The two XB-47s were not fitted with the tail turret as they were engineering and flight test aircraft, and in fact the prototypes weren't fitted with any combat gear at all.

Fuel capacity was an enormous 64,400 liters (17,000 US gal), compared to 19,000 liters (5,000 US gal) on the B-29. That meant that maintaining fuel trim to ensure a stable center of gravity in flight would be very critical.

The first prototypes was fitted with GE J-35 turbojet s, the production version of the TG-180, with 18 kN (3,970 lbf) thrust. Early jet engines did not develop good thrust at low speeds, so to assist in takeoffs in heavily loaded condition, the XB-47 prototype was to have 18 solid-fuel "jet-assisted take off ( JATO)" rockets with 4 kN (1,000 lbf) thrust each. Nine such units were built into each side of the lower rear fuselage, arranged in three rows of three bottles.

Similarly, the great weight of the aircraft and the lack of thrust reverser s made for high landing speed. Late in flight testing, following a suggestion by Air Force test pilot Major Guy Townsend, a 9.75 m (32 foot) drag chute was introduced to provide landing deceleration. A conventional parachute would be torn to shreds, so a ribbon parachute, derived from German designs, was used instead.

A related problem was that the aircraft's engines would have to be throttled down on landing approach. Since it could take as long as 20 seconds to throttle them back up to full power, the big bomber could not do a "touch and go" and circle around for another landing attempt if something went wrong. As a result, a second, half-diameter chute was added that would be deployed on approach so that the engines could be kept throttled up until the pilot felt sure his approach was safe.

The XB-47 was designed to carry a crew of three in a pressurized forward compartment: a pilot and copilot in a long fighter-style bubble canopy, and a navigator in a compartment in the nose. The copilot doubled as tail gunner, and the navigator as bombardier. The bubble canopy could pitch up and slide backward, but as the cockpit was high off the ground, crew entrance was through a door and ladder on the underside of the nose.

Total bombload capacity was to be 4.5 tonnes (10,000 pounds). Production aircraft were to be equipped with, by the standards of the time, advanced electronics for navigation, bombing, countermeasures, and turret fire control.

The second XB-47 prototype first took the air on 21 July 1948, and was equipped with much more powerful GE J47-GE-3 turbojet s with 23 kN (5,200 lbf) thrust each. The J47 or "TG-190" was a redesigned version of the J35 / TG-180. The first XB-47 prototype was later retrofitted with these engines.

Flight testing of the prototypes was particularly careful and methodical, since the design was so new in many ways. The prototypes initially suffered from " Dutch roll", an instability that caused the aircraft to weave in widening "S" turns. This problem was fixed by the addition of a " yaw damper " control system that applied rudder automatically to damp out the weaving. The prototypes also had a tendency to pitch nose up, and this problem was solved by tacking small vanes called " vortex generators" onto the wings that caused turbulence to prevent airflow separation .

Boeing test pilot Rob Robbins had originally been skeptical about the XB-47, saying that before the initial flight he had "prayed to God to please help me" through the flight. The aircraft was so unusual that he simply didn't know if it would fly. Robbins presently realized that he had an extraordinary aircraft.

In early 1948, the United States Air Force (having become a separate service in 1947) sent up a chase plane from Muroc (now Edwards) Air Force Base in California to help calibrate the bomber's airspeed system. Robbins reported later:

[The chase plane] was a P-80 [Lockheed Shooting Star] and Chuck Yeager was flying it. Chuck's a hell of a good pilot, but he had a little bit of contempt for bombers and a little disdain for civilian test pilots. Well, we took off, climbed out, and got up somewhere within four or five points of full throttle speed.

At that point, Chuck called me on the radio and said: "Bob, would you do a 180?" I thought: Hey, Chuck's smart, he just wants to stay reasonably close to Moses Lake, he doesn't have as much fuel as I do. Well, I turned around, got stabilized, and looked for Chuck. He wasn't there. Finally, I got on the radio and said: "Chuck, where are you?"

He called back and rather sheepishly said: "I can't keep up with you, Bob." So Chuck Yeager had to admit to a civilian test pilot flying a bomber that he couldn't keep up! That was something!

By mid-1948, the Air Force's bomber competition had already been through one iteration, pitting the North American " XB-45 " against the Convair " XB-46 ". The North American design won that round of the competition, and as an interim measure the USAF had decided to put the North American bomber into production on a limited basis as the " B-45 Tornado". The expectation was that B-45 production would be terminated if either of the remaining two designs in the competition, the Boeing XB-47 and the Martin " XB-48 ", proved superior.

The XB-47 was clearly an aircraft of enormous potential, but it was still so exotic that many USAF generals didn't take it seriously. At the end of July, USAF General K.B. Wolfe , in charge of bomber production, visited Boeing in Seattle, and Boeing president Bill Allen suggested that the general take a ride on the XB-47.

Wolfe was reluctant, but Allen and others managed to talk him into it, and Guy Townsend gave Wolfe an amazing ride. In early August, Wolfe contacted Boeing and indicated that the Air Force wanted to place an order for ten more of the new Boeing jet bombers. A formal contract was signed on 3 September 1948.

These ten aircraft were "B-47As". They were strictly evaluation aircraft. The first was delivered in December 1950. The configuration of the B-47As was close to that of the initial XB-47 prototypes. They were fitted with J47-GE-11 turbojet s, offering the same 23 kN (5,200 lbf) thrust as the earlier J47-GE-3, and they also featured the built-in JATO bottles.

Four of the B-47As were fitted with the K-2 bombing and navigation system (BNS), with and HD-21D autopilot, an analog computer, APS-23 radar, and a Y-4 or Y-4A bombsight. Two were fitted with the tail turret, one of them using an Emerson A-2 fire control system (FCS), another an early version of the GE A-5 FCS. The eight other B-47As had no defensive armament.

The B-47As were fitted with ejection seats. The pilot and copilot ejected upward, while the navigator ejected downward. Minimum safe ejection altitude was about 150 m (500 feet).

While the XB-47s had been built by Boeing at their Seattle, Washington, plant, the B-47As and all following Boeing B-47 production were built at a government-owned factory in Wichita, Kansas, where the company had built B-29s in the past. The switch was made as the Seattle plant was burdened with KC-97 production and other urgent tasks.

Most of the B-47As were phased out of service by early 1952, though one did perform flight tests for NACA for a few more years. While the Air Force put the B-47As through their paces, the Cold War was rising to full force, with a hot war flaming away in Korea. The USAF's Strategic Air Command (SAC) needed an effective nuclear deterrent to keep the Soviet Union in line, and the Stratojet was an excellent tool for the task, and Boeing was already working on production bombers.

2.2 B-47B

Following a series of preliminary contracts for production B-47s, in November 1949, even before the first flight of the B-47A, the Air Force had ordered 87 "B-47Bs", the first operational variant of the type. The first B-47B flew on 26 April 1951. A total of 399 were built, including eight that were assembled by Lockheed and ten that were assembled by the Douglas Aircraft Company, using Boeing-built parts.

The USAF was impatient to get their hands on as many B-47s as they could as quickly as possible, and signed up Lockheed and Douglas for the additional production. Lockheed-built aircraft were designated by a "-LM (Lockheed Marietta)" suffix and Douglas-built aircraft given a "-DT (Douglas Tulsa)" suffix. Boeing production was designated by a "-BW (Boeing Wichita)" suffix, except for the Seattle-built XB-47s and B-47As, which had a "-BO" suffix.

The initial batch of 87 B-47Bs featured the same J47-GE-11 engines as the B-47As, but all subsequent production featured substantially uprated J47-GE-23 turbojet s with 26 kN (5,800 lbf) thrust. Early production was retrofitted with the improved engines. They all featured the built-in JATO system used on the XB-47 and B-47A.

All featured full combat systems. Early production retained the K-2 BNS installed on some of the B-47As, but most production featured the K-4A BNS, which featured an AN/APS-54 warning radar and an AN/APT-5 electronic countermeasures (ECM) system.

The K-4A used a periscopic bombsight fitted into the tip of the nose of the aircraft, with the transparent plexiglas nose cone of the XB-47 and B-47A replaced by a metal nose cone. There were four small windows on the left side of the nose and two on the right. Another visible change from the earlier models was that the B-47B had a vertical tailplane with a squared-off top, rather than a rounded top as with its predecessors.

The bomb bay of the B-47B was shorter than that of the XB-47 and B-47A, since nuclear weapons had shrunk in the interim. However, the B-47B could carry a much larger bombload, of up to 8,165 kg (18,000 pounds). All B-47Bs carried the tail turret with twin 12.7 mm guns and the B-4 radar-guided FCS. The B-4 FCS proved troublesome, in fact so troublesome that some in some B-47Bs it was replaced with an N-6 optical sight. The copilot could swivel his seat around to face backward and sight the guns directly.

In practice, the enormous fuel capacity of the B-47 was still not enough to give it the range the Air Force wanted, and in fact there had been substantial prejudice against the type among senior Air Force brass because of the limited range of the initial design. Solution of this problem was a high priority, and so an " in-flight refueling (IFR)" receptacle was fitted in the right side of the nose for "boom"-style refueling. This was the main reason for getting rid of the plexiglas nose cone.

The B-47B was also fitted with a pair of jettisonable external tanks, carried between the inboard and outboard engine assemblies. These were really big drop tanks, with a capacity of 6,750 liters (1,780 US gal).

The B-47B suffered a considerable gain in weight compared to the B-47A, and so as a weight-reduction measure the ejection seats were deleted, and a windbreak panel was fitted to the aircraft's main door to make escapes easier. Some sources also claim that a fatal ejection-seat accident in a B-47A contributed to this decision. Whatever the case, this was not a very popular measure with crews, as getting out of the aircraft even at altitude was troublesome.

Bob Robbins recalled that George Martin, the B-47 program manager, showed him the letter from the Air Force ordering Boeing to remove the seats. Martin told Robbins: "Bob, put that letter in a safe place. The day is going to come when the Air Force is going to regret this decision, and we want to be able to make it very clear where the decision came from to do away with the ejection seats."

2.3 B-47E

The designations "B-47C" and "B-47D" were applied to special variants that never went into production (described later), and so the next production version of the B-47 was the definitive "B-47E".

The first B-47E flew on 30 January 1953. Four "blocks" or "phases" of the B-47E were built, each incorporating refinements on the previous block, and also sometimes featuring production changes within a block. Older blocks were generally brought up to the specifications of later blocks as they were introduced.

Early production "B-47E-Is" also known featured J47-GE-25 turbojet s with 27 kN (5,970 lbf) thrust, but they were quickly changed to J47-GE-25A engines, which featured a significant improvement in the form of water-methanol injection. This was a scheme in which a water-methanol mix was dumped into the engines at takeoff, increasing mass flow and so temporarily kicking the thrust up to 32 kN (7,200 lbf). The methanol was apparently just added to the water as anti-freeze. The engines left a trail of black smoke behind them when water-methanol injection was on.

In addition, although early B-47E-Is had the 18 built-in JATO bottles, they were quickly exchanged for an external, jettisonable "split v" or "horse collar" rack fitted under the rear fuselage. The rack carried 33 JATO bottles, in three rows of 11 bottles. The built-in JATO system was eliminated because of worries about having the JATO bottles so close to full fuel tanks, and in any case once the rocket bottles were exhausted they were just dead weight. The racks were expendable, and were dropped over specific range areas after takeoff.

Interestingly, B-47s rarely used JATO for takeoffs, as it was expensive and slightly more hazardous than a non-assisted takeoff. Apparently it was reserved for emergency alerts, when bombers had to get off the runway as fast as possible, and was otherwise only done once a year or so as a training measure. Water-methanol injection was a big help on takeoffs when JATO wasn't used.

The internal fuel capacity of initial production B-47Es was cut to 55,371 liters (14,610 US gal) as a weight-saving measure. This was considered acceptable because of the use of the big external tanks and the fact that the USAF had refined mid-air refueling to the point where it could be relied upon as a standard practice.

One particularly welcome change in the B-47E relative to the B-47B was the return of the ejection seats. Air Force brass had reconsidered the decision to delete them and realized it didn't make sense. In addition, the twin 12.7 mm guns in the tail turret were replaced with twin 20 mm cannon to provide more punch, backed up by an A-5 FCS in early production and an MD-4 FCS in later production.

A final change in the B-47E was that most of the windows in the nose were deleted, with only one left on each side. However, many pictures of B-47Es show them with the full set of windows used on the B-47B. Whether the number of windows varied through B-47E production, or whether these were B-47Bs updated to B-47E specification, is unclear.

The "B-47E-II" featured only minor changes from late production B-47E-Is. The "B-47E-III" featured an ECM suite, consisting of a radar jammer in a bulge under the fuselage plus a chaff dispenser, as well as improved electrical alternators.

The "B-47E-IV" was a much more substantial update, featuring stronger landing gear, airframe reinforcement, greater fuel capacity, and a bombload uprated to 11,340 kg (25,000 pounds), though the bomb bay was once again shortened because of the introduction of more compact nuclear weapons.

Another improvement was the introduction of the MA-7A BNS, a major step up from its predecessors. The MA-7A included the AN/APS-64 radar, with a range as long as 385 km (240 miles). The AN/APS-64 could be used as a long range " identification friend or foe (IFF) transponder" interrogator to allow a B-47E-IV to find a tanker or other B-47, or it could be used as a high-resolution ground-targeting radar. The B-47E-IV retained the optical bombsight, though this was rarely used.

BOEING B-47E-IV:

spec	metric	english
wingspan	35.4 m	116 ft
length	32.6 m	107 ft
height	8.5 m	27 ft 11 in
empty weight	35,860 kg	79,075 lb
max loaded weight	104,300 kg	230,000 lb
maximum speed	1,030 km/h	640 mi/h / 556 kn
service ceiling	10,100 m	33,100 ft
takeoff run	3,170 m	10,400 ft
range with tanks	6,495 km	4,035 mi / 3,510 nmi

A total of 1,341 B-47Es were produced. 691 were built by Boeing, 386 were built by Lockheed, and 264 were built by Douglas. Most B-47Bs were rebuilt up to B-47E standards. They were given the designation of "B-47B-II", though it appears that in practice they were simply called B-47Es.

2.4 RB-47E / RB-47H / ERB-47H / RB-47K

The B-47E was also the basis for a number of important long-range reconnaissance variants.

Boeing-Wichita built 240 "RB-47E" reconnaissance variants, similar to the B-47E but with a nose stretched by 86 centimeters (34 inches), giving them an arguably more elegant appearance than the bomber variants of the B-47. The long nose was used to stow up to 11 cameras, which could include:

* An O-15 radar camera for low-altitude work. * A forward oblique camera for low-altitude work. * A K-17 trimetrogon (three-angle) camera for panoramic shots. * K-36 telescopic cameras.

The RB-47E could carry photoflash flares for night reconnaissance. Although the RB-47E could be refueled in flight, its fuel capacity was increased, to a total of 70,000 liters (18,400 US gal). The navigator controlled the cameras, becoming a "navigator-photographer" instead of a "navigator-bombardier".

• A total of 32 "RB-47H" models were built for the electronic intelligence ( ELINT) mission, as well as three more specialized "ERB-47Hs". These aircraft featured distinctive blunt, rounded nose and sported blisters and pods for intelligence-gathering antennas and gear. They were designed to probe adversary defenses and then collect data on radar and defense communications signals.

The bomb bay was replaced by a pressurized compartment, which accommodated " electronic warfare officer s (EWOs)", also known as "Crows" or "Ravens". There were three Crows on board the RB-47H, but only two on the ERB-47H. A distinctive bulged fairing replaced the bomb bay doors. The RB-47H / ERB-47H retained the tail turret, and were also fitted with jammers and chaff dispensers. The only easily recognizable difference in appearance between the RB-47H and ERB-47H was that the ERB-47H had a small but distinctive antenna fairing under the rounded nose.

The first RB-47H was delivered in August 1955. The ELINT B-47s proved so valuable that they were put through a "Mod 44" or "Silverking" update program in 1961 to provide them with updated electronics systems. Silverking aircraft could be easily recognized by a large teardrop pod for ELINT antennas attached to a pylon, mounted under the belly and offset to one side of the aircraft, as well as an pylon-style antenna attached under each wing beyond the outboard engine. It is unclear if all RB-47Hs and ERB-47Hs were updated to the Silverking specification.

The RB-47H and ERB-47H were highly capable aircraft, but the EWO compartment was not only cramped, with sitting room only, but had both poor noise insulation and climate control. This made long missions very uncomfortable and tiring, and some sources say that the Crows even had to deal with fuel leaks on occasion. Getting out in an emergency was also a problem. The Crows had ejector seats that would literally cut a hole through the fairing in the bottom of the compartment when fired, but nobody regarded such a scheme as very credible.

Operations of the RB-47H and ERB-47H were top secret, with the missions generally flown at night and even base commanders often not knowing what they were all about. When crews were asked what they were doing, they always answered that such information was classified. On inquiries on what the blunt black nose was for, they would sometimes reply that it was a bumper, used in in-flight refueling in case they nosed into the tanker. This reply was often believed.

While a few of these aircraft performed special duties during the Vietnam War, such as relaying ELINT data from drones, they were eventually replaced by much more comfortable and capable Boeing RC-135 platforms. The last RB-47H was retired on 29 December 1967.

The final 15 RB-47Es built were fitted with additional equipment, including a "side looking airborne radar (SLAR)" system, and gear to sample the air for fallout from nuclear tests. The Air Force judged them different enough on delivery, beginning in December 1955, to give them a new designation of "RB-47K".

The RB-47Ks were generally used for weather reconnaissance missions, carrying a load of eight " dropsonde" weather sensors that were released at various checkpoints along the aircraft's flight path. Data radioed back from the dropsondes was logged using equipment operated by the navigator. The RB-47Ks stayed in service until 1963.

Incidentally, there were "F", "G", and "J" B-47 variants, but these were all one-shot conversions of B-47Bs or B-47Es, to be discussed later. There never was a "B-47I" variant. The Air Force didn't use the "I" suffix because it was too easily confused with "1".

3 B-47s in service

When B-47s began to be delivered to the Air Force, most crews were excited about getting their hands on the hot new bomber. The thing was so fast that in the early days, the B-47 set records everywhere it flew without even trying. The aircraft handled well and comfortably in flight, with a fighter-like light touch to the controls. The big bubble canopy also enhanced the fighter-like feel of the big aircraft, though it could make the cockpit a "hotbox" on sunny days, while the navigator in the nose shivered.

However, it took the Air Force until 1953 to turn the B-47 into an operational aircraft. The big aircraft was sluggish on takeoff and too fast on landings, a very unpleasant combination of circumstances. Furthermore, if the pilot put the machine down at the wrong angle on the bicycle landing gear, the aircraft would "porpoise", bouncing fore-and-aft. If he didn't lift off for another go-round, the instability would quickly cause the bomber to skid onto one wing and cartwheel to its destruction.

Improved training led to a good safety record, and few crews felt the aircraft was inherently unsafe or too demanding, but apparently there were aircrews who had little affection for or were even afraid of the B-47. Crew workload was also high, with only three crew members to keep the B-47 flying right. The B-52, in contrast, generally had six crew, with much less cramped accommodations.

The B-47's reliability and serviceability were also regarded as good. The only major problem was that the electronics systems were not very reliable, unsurprising given the technology available at the time. Much work was done to improve the reliability of the electronics systems, but they remained something of a maintenance headache all through the B-47's operational life.

Several models of the B-47 starting in 1950 included a fuel tank inerting system, in which dry ice was sublimated into carbon dioxide vapor while the fuel pumps operated or while the in-flight refueling system was in use. The carbon dioxide was then pumped into the fuel tanks and the rest of the fuel system, ensuring that the amount of oxygen in the fuel system was low, and thereby reducing the probability of an explosion in flight. Ten carbon dioxide tanks and heaters were involved. The system was implemented largely to reduce risks from static electricity discharges occurring during in-flight refueling.

By 1956 the US Air Force had 28 wings of B-47 bombers and five wings of RB-47 reconnaissance aircraft. The bombers were the first line of America's strategic nuclear deterrent, often operating at forward bases in the UK, Morocco, Spain, Alaska, and Guam. B-47 bombers were often set up on "one-third" alert, with a third of the operational aircraft available sitting on the runway, loaded with fuel and nuclear weapons, crews on standby, ready to take off for no-holds-barred attack against the USSR at short notice.

Crews were also trained to perform "minimum interval take-offs (MITO)", with one bomber following the other into the air at intervals of as little as 15 seconds, to get all the bombers on the way as fast as possible. MITO could be very hazardous, as the bombers left turbulence and, with water-methanol injection, dense black smoke that blinded pilots in the following aircraft.

Apparently B-47 bombers actually performed training missions in which they penetrated Soviet airspace in numbers. The facts behind these missions remain controversial, with some claiming that Curtis LeMay ordered them without presidential knowledge or approval.

The B-47 would be the backbone of SAC into 1959, when the B-52 began to take over and the B-47 wings started to be cut back. Actual B-47 production had ceased in 1957, though modifications and rebuilds continued after that.

Operational practice for B-47 bomber operations during this time went from high altitude bombing to low altitude strike, which was judged more likely to penetrate Soviet defenses. Bomber crews were trained in "pop-up" attacks, coming in at low level and then climbing abruptly on nearing the target before releasing a nuclear weapon, and the similar " toss bombing" procedure, in which the aircraft released the weapon while climbing and then rolled away to depart the target area before the bomb fell back down and detonated.

However, stresses due to low altitude operations led to a number of crashes, and an extensive refit program was initiated in 1958 to strengthen the wing mountings. The program was known as "Milk Bottle", named after the big connecting pins that were replaced in the wing roots.

The only B-47s to see anything that resembled combat were the reconnaissance variants. They operated from almost every airfield that gave them access to the USSR, and they often probed Soviet airspace. Sometimes they stirred up more trouble than they could handle. At least five of these aircraft were fired on, and three of these were shot down. The B-47s fired back with their tail turrets, though it is uncertain if they scored any kills, but it any case these were the only shots fired in anger by any B-47.

Final phaseout of B-47 bomber wings began in 1963, and the last bombers were out of service by 1965. The very last USAF operational aircraft was grounded in 1969. The US Navy kept specialized test aircraft in occasional use up to 1976. The final recorded flight of a B-47 was on 17 June 1986, when a B-47E was flown from the Naval Weapons Center at China Lake, California , to Castle Air Force Base , California, to be put in the air museum there. There are at least 15 B-47s that survive on static display, but none are still flying.

4 Conversions and special modifications

Aside from production aircraft, there were quite a number of conversions and oddball special modifications in the B-47 line.

B-47B conversions and special modifications included:

• The Air Force had considered building a specialized "RB-47B" reconnaissance variant to complement the B-47B bomber version, but as it turned out schedule slips and the like ensured that the RB-47E was the first production reconnaissance variant. As an interim measure before the RB-47E went into service, 24 B-47B bombers were fitted with a heated pod with eight cameras that was stowed in the forward bomb bay, and these aircraft were designated RB-47Bs. They were capable of daylight reconnaissance only.

• A total of 66 B-47Bs were also converted into "TB-47B" trainers, through the simple measures of adding a fourth seat for an instructor and removing the tail turret. These aircraft provided valuable crew training through most of the 1950s.

• With the introduction of the hydrogen bomb, the USAF contemplated the conversion of a few B-47Bs into "MB-47B" drones, which would essentially be huge cruise missiles carrying H-bombs. The program was known as "Brass Ring". Closer examination of the scheme showed that it was impractical, and Brass Ring was cancelled on the appropriate date of 1 April 1953.

• There were various flight tests through the 1950s for using the B-47B as a launcher for the big (9.5 meter / 31 foot) liquid-fueled " GAM-63 Rascal " missile, and one B-47B was modified to become a "YDB-47B" Rascal launcher.

However, the Rascal program was politically problematic, and never became operational, though a total of 74 B-47Bs were modified into "DB-74B" Rascal launchers before the program got the axe.

• In 1956, a single B-47B was converted into a "WB-47B" weather reconnaissance aircraft and operated by the Military Air Transport Service (MATS), making it one of the few B-47s that wasn't operated by SAC. This aircraft remained in service until the mid- 1960s.

• In 1953, two B-47Bs were modified for testing the probe-and-drogue refueling system. The tanker was given the designation "KB-47G" and was known as "Maw" by flightcrews, and was fitted with a British-built tanker kit. The refueling test aircraft was given the designation "YB-47F" and was known as "Paw", though other aircraft were also used as refueling targets.

The program was cancelled in 1954 as it turned out the KB-47G simply could not carry enough fuel to make it a useful tanker. The idea of fielding B-47 tanker conversions came up again a few years later, but the economics didn't make sense, and the notion was finally put to rest for good in 1957.

• One of the oddest B-47B conversions was the "CL-52", which was a B-47B loaned in 1956 to the Royal Canadian Air Force to test the big Orenda Iroquois turbojet for the Avro Canada CF-105 Arrow interceptor. Canadair attached the Iroquois engine to the right side of the rear fuselage near the tail, of all places, simply because there was no other place to put it.

Flying the CL-52 was reportedly a nightmare. After about 35 hours of engine flight tests, the Arrow project was cancelled in early 1959, and the B-47B / CL-52 was returned to the US. Some sources claimed it was bent out of shape by the tests, but in any case it was then scrapped.

The CL-52 was the only B-47, or for that matter the only US strategic jet bomber, to ever be used by any foreign service. However, in 1963, the USAF did demonstrate three B-47Es to Australian crews, as a possible stopgap before Australia acquired the FB-111. Nothing came of this exercise.

• The "YB-47C" (originally the "YB-56") was proposed by Boeing in 1950. It was to be a major variant, with four big Allison J71-A-5 turbojet engines, providing 45 kN (10,090 lbf) thrust each, in place of the six GEs J47s.

A B-47B was set aside to be converted to the prototype. The J71 engine program suffered setbacks, and so the Air Force decided to use the Pratt & Whitney J57 turbojet instead. However, the USAF then had second thoughts about the entire program, since by that time the future was clearly the B-52 and the B-47 was essentially an interim type, and gave up on the idea in December 1952.

• Beginning in 1951, two "XB-47Ds" were modified from B-47Bs as purely experimental platforms, with a big Wright YT49-W-1 turboprop engine spinning a huge four-paddle prop, replacing each of the inboard two-jet pods. Difficulties with engine development delayed first flight of the XB-47D until 26 August 1955. The aircraft's performance was comparable to that of a conventional B-47, and its reversible propellers shortened the landing roll, but the USAF did not follow up the idea.

• B-47E conversions and special modifications included:

• In 1955, a number of B-47E-Is were fitted with external pods, one mounted on either side of the bomb bay, with each pod containing four AN/ALT-6B jammers. The pods were known as "Tee Town pods" and so these aircraft were known as "Tee Town B-47s". They retained their normal bombing capability.

• The Tee Town B-47s then led to a specialized ECM conversion of the B-47E, which was given the designation "EB-47E". The initial EB-47 conversion featured a set of 16 jammers in a removable cradle stored in the bomb bay, plus radar warning receivers and chaff dispensers. These were known as "Phase IV" or "Blue Cradle" EB-47Es.

The more advanced "Phase V" EB-47E featured a pressurized module that was stowed in the bomb bay, with 13 jammers under control of two Crows. While the Phase IV jammer system was "broadband", blanketing a wide range of frequencies in hopes of jamming radars operating somewhere within that range, the Phase V jammer system could be selectively tuned to specific radar frequencies by the crows, permitting much higher jammer power on the frequencies that did the most good.

A radar jammer tends to announce its presence and location by the radio signals it emits, and EB-47E crews were perfectly aware that they were unlikely to return from an operational mission into the USSR. If they could cover for B-47 bombers, however, the sacrifice would be worth it. About 40 B-47Es were converted to B-47Es. Of course they couldn't carry bombs, but they did retain the tail turret.

• Two more B-47Es were converted to EB-47Es in the mid-1960s for US Navy service, on indefinite loan from the USAF. They were very much unlike the USAF EB-47Es, with some of their ECM gear fitted into pods carried on the external fuel tank pylons. They were used for tests of naval ECM systems and as "electronic aggressors" in naval exercises. These two aircraft were the last B-47s in service, and one performed the very last operational flight of a B-47 on 20 December 1977.

• Three B-47Es were converted to the highly specialized "EB-47E(TT)" configuration to be used for " telemetry intelligence ", picking up radio signals from Soviet missile tests and space launches. The EB-47E(TT)s featured a "Crow capsule" in the bomb bay loaded with the appropriate gear and two Crows, and also featured odd and distinctive "armrest" antennas just below each side of the cockpit.

All three of these aircraft were operated out of Turkey, and stayed in service until 1967. The armrest antennas attracted a good deal of attention from nosy personnel, and crews made up imaginative stories about them, for example claiming they were part of a "return to fighter (RTF)" defensive system that would cause Soviet air-to-air missiles to loop back and shoot down their own launch fighters.

• As with the B-47B, a few B-47Es were converted to trainers, with a fourth seat for an instructor, and given the designation "ETB-47E". These aircraft were used to replace TB-47Bs that had got too long in the tooth, and served into the early 1960s.

• Similarly, two B-47Es were converted to "YDB-47Es" to support the Rascal stand-off missile program, and two more B-47Es were converted to "DB-47Es" in preparation for the operational introduction of the missile before the program was axed. These two DB-47Es were later used as drone controller aircraft.

• Following the single WB-47B weather reconnaissance conversion, in the early 1960s 34 B-47Es were converted by Lockheed into "WB-47Es" for weather reconnaissance. These aircraft were stripped of combat gear, including the tail turret. They were fitted with cameras in the nose to take pictures of cloud formations, and carried a special meteorological instrument pod in the bomb bay. The last WB-47E was retired on 31 October 1969, and was the last B-47 in operational USAF service.

• In 1963, the Air Force modified 35 B-47Es to carry a communications relay system. These aircraft were given the new designation of "EB-47L", and were used to support US flying command post aircraft in case of a nuclear attack on the US. The EB-47Ls only remained in service for a few years, as improved communications technologies quickly made them redundant.

• A total of 14 B-47Es were converted to "QB-47E" target drones in 1959 and 1960. These aircraft were radio-controlled, and included such interesting features as self-destruct charges and arresting gear to assist in landings. They also carried pods mounted on the external tank pylons to help in scoring weapons tests. Apparently most of the missiles fired on them were directed for a near-miss, but the QB-47Es were nonetheless eventually whittled down to two survivors that were retired in the early 1970s.

• A single B-47E was modified to test the MA-2 BNS for the B-52, and given the designation "YB-47J". Other B-47Es were also apparently used in the MA-2 tests, but not given a special designation.

Several B-47Es were assigned to other specialized test duties and given the blanket designation of "JB-47E". One was used in the late 1960s to test "fly by wire" control system concepts.

Two B-47Es were also used for secret flight experiments in the early 1960s and given the designation "JTB-47E", and a third, even more mysterious modified B-47E was given the designation "JRB-47E". They seem to have been test platforms for ECM systems.

The final RB-47H to be retired from service was later pulled out of the "boneyard" and used for tests of avionics for the General Dynamics F/B-111. This RB-47H was fitted with an F-111-style nose and served flew into the early 1970s. It was not given any special designation.

Finally, a B-47E was loaned to the US Navy to help test the GE TF-34-2 turbofan for the Lockheed S-3 Viking carrier-based antisubmarine warfare aircraft. This B-47E was given the designation "NB-47E" and performed test flights from 1969 through 1975.

5 Perspective

The total number of B-47s built by Boeing's count was 2,042, of the following models and quantities:

XB-47: 2 B-47A: 10 B-47B: 399 B-47E: 1,341 RB-47E 240 RB-47H: 32 ERB-47H: 3 RB-47K: 15

The B-47 undeniably fulfilled its design objectives. Perhaps as importantly, it led directly to the Boeing B-52, which

• still survives as an important US military asset, and
• in turn provided much inspiration for the design of
  • Boeing's KC-135 air-to-air refueling tanker, which survives prominently in Air Force service as of 2003, and
  • the Boeing 707, the model for the modern commercial jetliner.

The B-47 is very elegant even by 21^st-century standards, but considering what most of its contemporaries looked like, it must have appeared revolutionary when it was introduced. Most bomber variants were painted white on the underside to reflect the flash of a nuclear explosion, and this color scheme helped give the aircraft a clean, elegant appearance.

One B-47 is at the Museum of Flight in Seattle, currently being reconditioned for display.

6 References

• B-47 STRATOJET: BOEING'S BRILLIANT BOMBER by Jan Tegler, McGraw-Hill, 2000. The v2.0.0 version owes much to this book.

• THE BOEING B-47 by Peter M. Bowers, Profile Publications, 1966. I recall having purchased this document in a hobby shop in the late 1960s, and was pleased to find another copy of it in a slightly tattered set of Profiles at the local library in 1996.

• "Stratojet!", by Robert C. Dennison, AIR COMBAT, July/August 1997. Dennison is a retired USAF officer and B-47 pilot who works with the B-47 Stratojet Association.

• "The Long Reach Of The Stratojet", by Walter J. Boyne, AIR FORCE MAGAZINE, December 1997, 66:71.

The initial version of this article was based on a public domain article from Greg Goebel's Vectorsite .

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Designation Series	XB-44 - B-45 - XB-46 - B-47 - XB-48 - YB-49 - B-50
Related Lists	List of military aircraft of the United States - List of bomber aircraft

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