Thisfighter - by far the most famous of all Japanese aircraft - dominated the first six months of the aerial war in the Pacific, and continued in service until the end of hostilities. The Zero - allied code-name 'Zeke' - was remarkable in being the first carrier fighter to outperform its land-based equivalents.  It had been designed by Mitsubishi to meet the severe demands of the 1937 Imperial Navy specification for a shipborne fighter - demands which included a speed of 500 km/h (311 mph) and an armament (powerful for the time) of two cannon and two machine-guns.  The result was a small, lightly-built aircraft with outstanding maneuverability. 

The first production version received a more powerful engine than the prototype and was designated the 'A6M2'.  As it was first produced in 1940 - the Japanese year 5,700 - it became popularly known as the "Zero-Sen" ("Type 00 Fighter").  Two squadrons with 15 planes were sent to China in July 1940 for trials under operational conditions, and quickly eliminated all opposition.  The effectiveness of the Zero was urgently and emphatically reported to Washington by General Chennault, commanding officer of the Flying Tigers, but his report appears to have gone unnoticed. 

More than 400 A6M2 and A6M3 (clipped-wing) Zeros had been delivered by the time of the attack on Pearl Harbor. 
They had unprecedented range for a small aircraft, and their unrivalled maneuverability and powerful armament gave them decisive superiority over every opponent. This was startlingly demonstrated in the heavy Japanese carrier raids on Ceylon 5-9 April 1942, in which they were in action against British Hawker Hurricanes.  These were fighters which were regarded as outstandingly maneuverable(and which had proved their superiority in this respect to the Messerschmidt Bf-109s of the German Luftwaffe). Nonetheless the Hurricanes were easily out-turned by the raiding Zeros. 

Nonetheless, in the great carrier battles of Coral Sea and Midway in mid-1942,  in which the Zero encountered Grumman F4F Wildcats flown by some of the US Navy's most able pilots,the weaknesses of the Japanese fighter began to show. 
The A6M's lack of armour and self-sealing fuel tanks, combined with the general lightness of its contruction,  made it exceptionally vulnerable to combat damage. 

At Midway many pilots were lost aboard the Japanese carriers,all four of which were set ablaze by the carrier-borne American Dauntless dive-bombers. In the protracted and bitter Guadalcanal campaign losses of aircrew mounted and the quality of Japanese pilots correspondingly declined.  Allied aircraft therefore achieved increasing success against the Zero. When, on top of this, much more modern and capable US aircraft - notably the Grumman F6F Hellcat and Vought F4U Corsair - appeared in the combat areas of the Pacific,  the A6M found itself outclassed. 

Mitsubishi therefore tried desperately to come up with a more effective version of the Zero. These efforts resulted in the A6M5 - the variant produced in the largest numbers.  However, the improvement it represented was not sufficient, and the Zero was never, after 1943, able to fight on equal terms with the best Allied aircraft.  However,  the A6M6c equipped with the combat-boosted Sakae 31 engine,  and the A6M8c equipped with 1,560 hp Kinsei 62 engine, were in 1945 able to give considerable trouble to the F4F and FM Wildcat fighters operating from US escort carriers.

At the Battle of the Philippine Sea 220 or so of the Japanese Mobile Fleet's 430 carrier aircraft were Zeros -  many of them operating as bombers.  A6Ms were again in action at the Battle for Leyte Gulf, mainly as attack aircraft, and from October 1944 until the end of the War Zeros were employed in hundreds of kamikaze attacks on American warships. 

Total production of the A6M came to 10,449 units.

A6M Data

Origin
Mitsubishi Jukogyo KK

Type
Single-seat shipborne fighter and fighter-bomber

Dimensions
Span (A6M1, A6M2) 39' 4" (Other versions) 36' 1"
Length  29' 9" /  Height 9' 8 - 9' 9"

Weights
(A6M3)  3,984 lb empty,  5,828 lb
(A6M5)  3,920 lb empty,  6,050 lb loaded

Engine
(A6M3 & A6M5)  One 1,030 hp Sakae 21 radial
(A6M5 had individual exhaust stacks as in illustrations above, giving slight boost)

Armament
2 x 20 mm Type 99 cannon in outer wings
2 x 7.7 mm or 13.2 mm machine-guns above forward fuselage or in wings
wing racks for 2 x 66 lb (or in some aircraft 132 lb) bombs

Maximum speed
  (A6M3) 336 mph  (A6M5) 354 mph  (A6M8c) 360 mph

The A6M - Origins and Development
 

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.

The A6M3 was ordered into production as the Navy Type 0 Fighter Model 32, and was followed in early 1942 by the A6M2-K two-seat trainer based on the A6M2.  Operational experience showed that the A6M3 was inferior in range to its predecessor. Thus there was a reversion to the full-span wing with folding tips with the A6M3 Model 22. 

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 A6M5 was given an armament of  two 20mm Type 99 cannon and two 7.7mm machine-guns. Sub-variants included - the A6M5a, with a modified wing to accept belt-fed instead of magazine-fed cannon;  the A6M5b which had one of the 7.7mm machine-guns replaced by a 13.2mm machine-gun and introduced armoured glass in the cockpit canopy;  the A6M5c with three 13.2mm machine-guns and two 20mm cannon;  the A6M5d-S night-fighter version with an obliquely-mounted 20mm cannon in the rear fuselage;  and the A6M5-K two-seat trainer. 

Development continued with the conversion of the A6M5 airframe to A6M6 standard by the installation of the Nakajima Sakae 31 engine with water-methanol injection, making possible a maximum speed of 345 mph (555 km/hour). Production A6M6 Zeros also introduced self-sealing fuel tanks. Operational units carried out field conversions on many to take a 551-pound (225-kg) bomb on the drop-tank mounting, enabling them to be used as fighter-bombers.  The final production version - the A6M7 - which entered service in mid-1945, was specifically designed for this role.  In addition to the installation of a bomb-rack it had a reinforced tailplane and underwing attachments for two drop-tanks.

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.

Acknowledgments

Main source for the  above data was Bill Gunston's 'Combat Aircraft of World War II' (Published by Spring/Salamander - London 1978), The profile drawing of the A6M2 of the 'Hiryu' air group at the head of this page is based on an illustration in the same work The other illustrations on this page are reproduced from 'Axis Aircraft of World War II' by David Mondey  (Chancellor Press, 1996). I am most grateful to the authors and publishers of these works +++