Taking the experiences of the A5M in China the IJN sent out updated requirements in October requiring a speed of 270kn (310 mph/500 km/h) at 4.000 m (13.000 ft) high and a climb to 3.000 m (9.800 ft) in 9.5 minutes. Equipped with drop tanks, they required an endurance of two hours at normal power, or six to eight hours at economical cruising speed. Required armament consisted on two 20 mm cannons, two 7,7 mm machine guns and two 60 kg (130 lb) bombs. Every aircraft was also going to be equipped with a complete radio set, together with a radio-direction finder for long range flights. Manoeuvrability was required to be at least equal to that of the A5M and the wingspan had to be less than 12 m (39 ft) to allow the fighter to operate from aircraft carriers.
Nakajima's design team considered the requirements impossible to achieve and pulled out from the competition in January. On the other hand, Mitsubishi's chief engineer, Jiro Horikoshi considered that the requirements could be met but only if the aircraft was made as light as possible. Therefore, every possible weight-saving measure was incorporated into the design. I fact, most of the aircraft was built in a new top-secret material called 'extra super duraluminium' which was developed in 1936 by Sumitomo metal industries. This material was lighter, stronger and more ductile than other alloys used at the time, but it was also more prone to corrosion making it brittle. This defect was countered with an anti-corrosion coating applied after fabrication. This design had no armour protection provided for the pilot, engine or other critical points and self-sealing fuel tanks, which were starting to be popular at the time, were not provided. Those features made the A6M more manoeuvrable, lighter and the longest-ranged single-engined fighter of the World War II, making the fighter capable of searching for an enemy, hundreds of kilometres away, bringing it into battle and then returning into its base or aircraft carrier. However, given its lightness it was also very prone to catch fire and explode when hit by enemy fire.
Given its low-wing cantilever monoplane layout, retractable, wide-set conventional landing gear and enclosed cockpit, the A6M was the most advanced carrier fighter at the time of its introduction. It had a fairly high-lift, low-speed wing with very low wing loading. This, mixed with its light weight, resulted in a very low stalling speed of well below 60 kn (69 mph 110 km/h) which made the main reason for its excellent manoeuvrability, allowing to out-turn every Allied fighter at the time. Early models featured servo-tabs on the aillerons after pilots complained that control forces were too heavy at speeds above 300 km/h (190 mph). They were discontinued on later variants after discovering that the lightened control forces were causing pilots to over stress the wings during vigorous manoeuvres.
It has been claimed that its design show clear influences from British and American fighters and components exported to Japan in the 1930s, and more in particular to both the Vought V-141 (whose prototype and blueprints were sold to Japan in 1937) and the British Gloster F.5/34.
While the resemblance to the V-141 seems debatable, its obvious that the Japanese took some inspiration from it. However, considering that performances of early models are identical in numbers to those of the Gloster F.5/34, apart from the fact that its dimensions and appearance are also equal and that Gloster had relationship with the Japanese during the Interwar period (Nakajima manufactured the Gloster Gambet under license), it seems that the F.5/34 is the most direct inspiration for the A6M.
Its usually known as the 'Zero' as its Japanese Navy designation was called as 'Type 0 carrier fighter' as it was taken from the last digit of the imperial year 2.600 (1940) when it entered service. It was most commonly called by the pilots as the 'Zero-sen' while the Allied code-name was 'Zeke'.
Sources:
1. https://en.wikipedia.org/wiki/Mitsubishi_A6M_Zero
2. Signal Squadron - Aircraft In action 59 - Mitsubishi A6M Zero in Action
3. https://www.valka.cz/Micubisi-A6M-Reisen-Zeke-Zero-t15680
4. Salamander Books - The Complete Book of Fighters
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