The Mitsubishi Zero (Type 0 Fighter) was designed by Jiro Horikoshi for the Imperial Japanese Navy to replace the A5M which had, in successive forms, been the service fighter since 1936. The Zero was a cantilever low-wing monoplane of all-metal construction except for tail and control surfaces, which were fabric-covered. The prototype was acceptable except for its top speed and so a 925-hp Nakajima NK1C Sakae 12 radial engine was installed and the designation A6M2 was given to the aircraft. Fifteen aircraft were supplied for evaluation in China and the American Claire Chennault was so impressed by their performance that information was sent to USAAF headquarters on them. It appears to have been ignored, as was most of the intelligence emerging from China on Japanese military matters by both Americans and British. By the time of the attack on Pearl Harbor more than 400 aircraft had been delivered to the navy. The majority were the A6M2 Model 11 with two .303-in machine guns in the fuselage and two 20mm Type 99 cannon in the wings, while the rest were A6M2 Model 21 which had manually-folding wingtips to aid carrier stowage. The Zero was given the codename "Zeke" by the Allies. The 1st Fighter Combat Unit, 2nd Fighter Combat Unit, 3rd Fighter Combat Unit, 4th Fighter Combat Unit, 5th Fighter Combat Unit and 6th Fighter Combat Unit of the First Air Fleet were made up entirely of Zeros.
You can find out more about Japanese aircraft in the following Osprey books: