The new fighter - the A6M1 - was a cantilever low-wing monoplane of all-metal construction, except for fabric-covered tail control surfaces.  It had retractable landing gear, an enclosed cockpit directly over the wing, and in its prototype form was powered by a 780 hp Mitsubishi Mk2 Zuisei (auspicious star) radial engine. 

It flew for the first time on 1 April 1939.  Flight testing showed that this and a second prototype first flown in October 1939 more than satisfied the requirements of the Navy specification, except in respect of maximum speed.  To overcome the shortcoming in speed the Navy requested that the design be modified to incorporate as powerplant the 925 hp Nakajima NK1C Sakae (prosperity) 12 radial engine. Although somewhat larger and heavier than the Zuisei, the Sakae engine was installed in a third prototype, which was given the company designation A6M2.  First flown on 18 January 1940 the A6M2 proved so successful that, following service trials of this and a similarly-engined fourth prototype, on 21 July 1940 the Navy arranged with Mitsubishi for the supply of 15 pre-production A6M2s for operational evaluation in China. On the last day of July 1940 the A6M2 was ordered into production under the official designation Navy Type 0 Carrier Fighter Model 11. 

The pre-production Zeros sent to China were extraordinarily successful. Despite their small numbers they were used extensively, and as the first production aircraft became available they were quickly reinforced. Large numbers of opposing aircraft were destroyed, with only a handful of A6M2s lost - to defensive fire from the ground. 

Mitsubishi's manufacture of A6M2s was supplemented by Nakajima, a production line being established at that company's Koizuma factory, and by the time of the attack on Pearl Harbor more than 400 Zeros had been delivered to the Navy. These comprised the initial production Model 11 and the Model 21 which differed by having manually-folded wingtips for stowage aboard aircraft carriers. 

Six months before the Pacific War started Mitsubishi had flown the prototype of an improved version - the A6M3 - which introduced the the 1,130 hp Nakajima Sakae 21 engine with a two-stage supercharger, and which had clipped wings to avoid the production complication of the A6M2 Type 21's folding wingtips.

By 1943 encounters with the new Allied fighters made it imperative that the Zero's performance be drastically improved.  This was to result in the major production version, the A6M5. It was preceded by two A6M4 prototypes which were conversions from A6M2s, powered by an experimental turbocharged Sakae engine to give better performance at high altitude.  However, problems with this engine forced Mitsubishi to retain the existing Sakae 21 for the A6M5, which was ordered into production as the Navy Type 0 Fighter Model 52. This version had a redesigned clipped non-folding wing with rounded tips, and introduced individual exhaust stacks on the engine to give increased thrust. 

The A6M8c represented an attempt to extend the operational life of the Zero, two prototypes converted from A6M7 airframes having a redesigned and strengthened forward fuselage to enable installation of the 1,560 hp Mitsubishi MK8P Kinsei 62 radial engine. Service testing showed this to be a formidable fighter, and some 6,300 were ordered - but none 
was completed before the end of the war. 

A total of 10,449 Zeros were built - 3,879 by Mitsubishi and 6,570 by Nakajima.