Designed to meet the incredibly high bar made by the Japanese navy (an aircraft capable of 500mph, two 25mm cannons, and two 7.5mm machine guns, bombing ability), the A6M was originally designated the “Hamp” by the Allies, before being called the “Zeke”, then finally settled on “Zero”, because of the Japanese designation “Navy Type Fighter 00 (or double-zero)”.
Due to the aforementioned requirements, lead designer Jiro Horikoshi made the aircraft as light as possible (a new super-light alloy was created just for this aircraft), stripping all protection away from the pilot, the engine, and all other critical points of the plane. Drop tanks' were added for ones intended for long journeys.
Testing was done in the early '40s over China, where a confirmed 269 Chinese aircraft were shot down by a single force consisting of 15 A6Ms.
The first major engagement that A6Ms were involved was the attack on Pearl Harbor, making up the bulk of the Japanese force.
The planes were superior in almost every way to equivalent Allied aircraft of the time, though it severely lacked resilience; a A6M could use it’s ammunition on one F4F, and still have the F4F be capable of flight, while the A6M itself could only take a small amount of damage before being rendered inoperable.
Still, its speed and great numbers put it in a favorable position against other Allied aircraft (at least during the mid-years of the war), making it useful as a fighter, and, once again, its speed made it excellent for use in hit and run attacks’, and several later Japanese as well as Allied planes began incorporating features from it.
Unfortunately, its one advantage on the field (speed), was lost when the Allied Grumman F6F Hellcat, Vought F4U Corsair and Lockheed P-38’s were brought to the pacific theatre of the war (the above-mentioned aircraft were partially designed as a retaliatory response to the A6M).
Because of its lost superiority, remaining A6Ms were used for the last part of the war as kamikaze suicide bombers, due to there still respectable velocity.