The Mitsubishi A6M2 Reisen (Zero) went into production on July 31, 1940 as the Navy Type 0 carrier fighter, Model 11.
Nakajima engineers felt that the design specifications were unachievable so they pulled out of the design competition which left only Mitsubishi. The Mitsubishi's design team was headed by Jiro Horikoshi and they came up with the A6M1 prototype. Careful attention was paid to weight savings, and a new special aluminum alloy developed by Sumimoto was used through out the airframe. The first prototype A6M1 did not achieve the air speed and rate of climb required by the design specifications so the Mitsubishi 780 hp Zuisei 13 engine was replaced by the 14 cylinder twin row radial Nakajima NK1C Sakae 12 and the aircraft re-designated as the A6M2. Initial test trials were completed on the prototype in July 1940 and production started immediately.
The A6M2 achieved great success early in the war as it easily out performed all of the allied aircraft in the Pacific theater. It was not until June of 1942 when a crash landed but virtually undamaged A6M2 was recovered in the Aleutian islands that the US was able see and fly, first hand, this advanced fighter aircraft to see where its vulnerabilities were.
The A6M2 was replaced in service by the A6M3 (Hamp) Navy Type 0 Carrier Fighter Model 32 in early 1943 which was powered by the more powerful Nakajima Sakae 21 engine with a 2 speed super charger. The folding wing tips which were necessary for the A6M2 fighter to fit on the Carrier elevators were replaced by a shorter squared off wing. For more information about the A6M-3 Hamp you can read a reprint of the in-depth Aviation Magazine article on this fighter aircraft. The A6M2 aircraft which remained in service were were relegated to second line duty and used as training aircraft.
By late 1942, early 1943 the US had introduced the Navy F4U Corsair and the Air Force P-38 into the Pacific theater. All of these new US aircraft were an equal match for the A6M3 and because of their heavy pilot armor and fuel tank armor they could take a lot of punishment and still be an affective adversary. The A6M3's ineffective dive speed was being used to the advantage of the American pilots who found that they could engage the Zero and dive away to relative safety without fear of being followed.
By June of 1944 the newly redesigned A6M5 was in full production and had been deployed as the Navy Type 0 Carrier Fighter Model 52. It now had a top speed of 351 mph and could dive at speeds of 410 mph thus eliminating the faults of its predecessors. The first major engagement in which this aircraft took part was the the Marians (Philippine Sea) on June 19th 1944 where it was engaged by the newly outfitted US carrier F6F Hellcat fighter aircraft.
The A6M Reisen was the first aircraft to be used in Kamikaze attacks against the US fleet on October 25, 1944 which was carried out by Japanese Air Group 201 and launched from bases in the Philippines.
Over 10,900 A6M2 through 5 Reisen Zero's were produced during the war by Mitsubishi and Nakajima and it was the most widely encountered Japanese fighter aircraft. The A6M remained in production up until the final day of the war.
The main failing of the A6M2, 3, and 5 fighter aircraft were the lack of armor for the pilot and its fuel tanks were not self sealing and once breached the aircraft was lost.
Up until the introduction of the P-38 Lightning, the F6F Hellcat, and the F4U Corsair the A6M Zero was the premier fighter aircraft in the Pacific. It could out maneuver anything in the sky and with it's twin 20 mm cannons and twin 7.7 mm machine guns in the hands of a skilled pilot it was a terrifying and worthy adversary.