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While the official Allied code name was Zeke, it is universally known as Zero from its Japanese Navy designation, Type 0 Carrier Fighter (零式艦上戦闘機), taken from the Imperial year 2600 (1940). It was also known as a Tojo Zero.
A combination of excellent manuverability and very long range made it the best fighter of its era and gained it a legendary reputation, but design weaknesses and lack of more powerful engines eventually doomed it.
The Mitsubishi A5M was just starting to enter service in early 1937 when the IJN started looking for its eventual replacement. In May they issued specification 12-Shi for a new carrier based fighter, sending it to Nakajima and Mitsubishi. Both started preliminary design work while they awaited more definitive requirements to be handed over in a few months.
Based on the experiences of the A5M in China, the Navy sent out updated requirements in October. The new requirements called for a speed of 500 km/h at 4000 m, and a climb to 3000 m in 3.5 min. They needed an endurance of 2 hours at normal power, or 6 to 8 hours at economical cruising speed (both with drop tanks). Armament was to consist of two 20 mm cannons and two 7.7 mm machine guns, and two 30 kg or 60 kg bombs. A complete radio set was to be mounted in all planes, along with a radio direction finder for long-range navigation. Finally the maneuverability was to be at least equal to A5M, while the wing span had to be less than 12 m to fit on the carriers.
Nakajima's team thought the new requirements were ridiculous and pulled out of the competition in January. Mitsubishi's chief designer, Jiro Horikoshi, felt that the requirements could be met, but only if the aircraft could be made as light as possible. Every weight saving method was used, and the designers made extensive use of the new duralumin alloy. With its low-wing cantilever monoplane layout, retractable wide-set landing gear and enclosed cockpit, the design was not only much more modern than any the Navy had used in the past, it was one of the most modern in the world.
At the time of Pearl Harbor there were only 420 Zeros active in the Pacific. The carrier borne Model 21 was the type encountered by the Americans, often much further from its carriers than expected, with a mission range of over 1600 miles (2,600 km). They were superior to all current Allied fighters in the Pacific and remained unchallenged until early 1943, although in competent hands the Zero was still deadly at the end of the war. Because of their reputation and ease of manufacture the Zero remained in production until the end, with over 11,000 of all types produced.
Designed for attack the Zero gave precedence to manoeuvrability and fire-power at the expense of protection - most had no self-sealing tanks or armour plate - thus many Zeros were lost too easily in combat. Nevertheless, many Allied pilots died trying to learn how to fight such an agile aircraft.
It was not until new tactics were developed that US pilots were able to fight the Zero with any measure of equality. The correct combat tactic against Zeros was to remain out of range and fight on the dive and climb. By using speed and resisting the deadly error of trying to out-turn the Zero, eventually cannon could be brought to bear and a single burst of fire was usually enough. Another important maneuver was called the "Thatch Weave," named for the man that invented it. It required two planes, a leader and his wingman, to fly about 200 feet apart. When a Zero would latch onto the tail of one of the fighters, the two planes would turn toward each other. If the Zero followed its original target through the turn, it would come into a position to be fired on by his target's wingman. This tactic was used with spectacular results at the Battle of the Coral Sea and at the Battle of Midway, and helped make up for the inferiority of the US planes until new aircraft types were brought into service.
When the US had learned the "secret" of the Zero new aircraft such as the Grumman Hellcat and Vought Corsair were introduced, planes that outperformed the Zero in every way but maneuverability. To correct that shortcoming, US pilots had to apply the correct tactics. The result was that the US Navy's 1:1 kill ratio suddenly jumped to better than 10:1. However Japanese development did not remain static - newer planes like the George were excellent fighters and a match for the later US models.