The Japanese planned the Aleutian islands assault to be at the same time as the Midway battle. During this unsuccessful operation, one A6M2 made an emergency landing on the island. The pilot died and the US captured the aircraft. It was hardly damaged. Another airplane was captured in China. The tests helped developing the hit-and-ran tactics for the US pilots and sped up the production of the Hellcat. A6M2 was inferior to Grumman F6F-3 carrier borne fighter in speed, climb rate and survivability. Later on the US would study the trophy A6M3,A6M5 and (after war) A6M7 with the same attention to detail.

Captured A6M2 Model 21 during tests in the US, 1943	Captured A6M3 Model 32 during tests in the US, 1943-44

A6M3 Model 32

Horikoshi himself understood the shortcomings of Zero perfectly. The model was under constant modification: longerons were reinforced, covering was amplified, etc. Model 21 already had trimmers which deviated in sides opposite to the ailerons, which reduced force on handle at high speeds (at low speeds the trimmers were blocked by the mechanic drive of the landing boards). Cardinal decision was applied for the next Model 32 (A6M3), tested starting with June of 1941 and produced from June, 1942 on: trimmers were removed from the trapezoid wing, the ailerons area was reduced, the covering reinforced. Cannon drums were abandoned in favour of clips, which allowed for higher load. Most importantly, the aircraft was fitted with the Nakajima Sakae-21 engine with a two-stage supercharger, offering 1130 hp for take-off. A6M3 became even more superior to the US F4F-4 Wildcat in speed and climb rate. Unfortunately, the fuselage fuel tank capacity had to be reduced (as Sakae-21 was longer than the previous engine), and the increased fuel expenditure shortened the battle range of the aircraft. A6M3 could not support bombers in Guadalcanal raids even with an extra fuel tank under fuselage. Therefore, in December of 1942 Model 22 was put into production. Its wing construction was similar to that of Model 12, and possessed higher wing tanks capacity. The battle range grew, but other characteristics fell. The new US Army and Navy fighters (Lockheed P-38 Lightning, adopted since autumn of 1942, and Chance-Vought F4U Corsair, adopted since February of 1943) by far surpassed Zero in horizontal speed, especially at middle and high altitudes, as well as in dive. The battle of April 18, 1943, in which the Japanese Imperial Fleet Commander-in-Chief Admiral Yamamoto died, was very typical of this. The Staff of the Japanese fleet was inspecting the Rabaul troops on two Mitsubishi G4M Betty bombers supported by 9 Zero fighters. The US Commandment had acquired the key to the Japanese codes and knew the time and route of the flight. The group consisting of 14 P-38 intercepted the Japanese aircrafts, downing both Betties and two Zero at the cost of only one Lightning.

Hiroyoshi Nishizawa's A6M3 Model 22 (86 kills scored) in a sortie over the Solomon islands, summer 1943	A6M5 Model 52 during tests in the US, 1945

The attempt at creation of A6M4 with an extra turbo-compressor for the interception of heavy high-altitude US bombers and high-altitude fighters failed. The next modernization was more successful. Model 52 (A6M5), designed in August of 1943, had integral wings of reduced span (11m), reinforced covering and individual jet exhaust branch pipes of the engine. The horizontal and dive speed increased without lowering agility and climb rate (the previous Zero models had, in fact, too low wing load). In autumn of 1943, when A6M5 was massively used in combat, the next Model 52-co (52a) was brought to tests. It had even more reinforced wings and cannons with cartridge belts of increased capacity. However, the first batch of A6M5a's was produced by Mitsubishi and Nakajima only in February of 1944. By that time the US Navy was already equipped with F6F-5 Hellcats and F4U-1D Corsairs, which surpassed all of Zero's parameters. During “the Great Marianas Turkey Shoot” on June 19, 1944, the A6M5 model suffered a complete defeat by the Hellcat: the US fighters downed over 300 Japanese aircraft, losing only 30 machines. Both Japanese bombers and Zeroes burned at the impact of just a few 12.7-mm incendiary Colt-Browning bullets. Slow 20-mm cannons and 7.7 mm sub-machine guns of the Zero could not do harm to the hardy F6F-5, even when those planes accidentally got into Japanese sights, roaring past at an incredible speed.

A6M5 Model 52 cockpit. Good view of breech-loaders of 7,7mm guns with manual reload.

A6M5b Model 52 «Otsu», with an additional fuel tank, 1945	A6M5c Model 52 «Hei» with an additional fuel tank and 13.2mm guns, 1945

The Japanese still had no aircraft which could replace Zero, and manufacturing of A6M5 modifications continued in large quantities. Zero became the most mass-produced Japanese aircraft of the Second World War. Model 52-otsu (52b) passed the tests in April of 1944 and was put into production in June. The designers made an attempt to increase armament and survivability of the machine, equipping it with armored glass, carbon dioxide fire extinguishing system, and changing the starboard fuselage low-caliber machine gun for the powerful 13.2 mm 13 mm Type 3. These improvements, just like all the following ones, increased the weight of the aircraft and decreased its flight characteristics. The destructive assaults of Boeing B-17 Flying Fortress and Boeing B-29 Superfortress, as well as the incapability of Japanese interceptors to shoot them down, asked for additional modifications of the Zero. Model 52-hei (52c), put into production in October of 1944, was supposed to become the “bombers' bane”. Wings were fitted with additional 13.2 mm machine guns, while the front 7.7 machine gun was removed. The cabin was reinforced with armored back shield and panel. The fuel capacity of the aircraft decreased regardless of the fourth, protected tank installed in the fuselage. The Japanese developed 150 mm air-to-air missiles, two of which could be carried under the wings of the plane. Nevertheless, the new A6M5c did not have much success in combating Fortresses due to insufficient engine power for the heavy aircraft, especially at high altitudes. The armament was also quite weak. The attempt to introduce the forced Nakajima Sakae-31 with water-methanol injector in November of 1944 went wrong due to constant engine failures, and Model 53-hei (A6M6c) did not make it into production.

The early Zero models had already come equipped with low-caliber bomb holders, but by autumn of 1944 the necessity of transforming Zero into a fighter-bomber became quite evident. Japanese dive and torpedo bombers were not able to approach US vessels and drop the bombs before being shot down by fighters and AA guns. In a desperate attempt to change the outcome of the war, the Japanese began forming kamikaze units. The first group of kamikazes was formed at the Lawson airfield in Philippines in October of 1944. It was equipped with old training A6M2's and new А6М5 aircraft, all fitted with 250 kg bomb holders under fuselage. The training tests showed that the construction and the cockpit had to be reinforced in order to give the airplane a chance to make it through the incoming fire. In spring of 1945 the new A6M7 fighter-bomber was put into production. It was capable of carrying up to a 500 kg bomb under the fuselage and a pair of 150-liter fuel tanks under the wings. The aircraft had strengthened rear, protected fuel tanks and armament similar to that of A6M6c (lacking front machine guns). The wing 13.2 mm machine guns were often dismounted. The Sakae-31 engine was finally ready for production. Unfortunately, by that time, the manufacturing had been disorganized by a strong earthquake and bombings. The manufacture choice between A6M5c Mod.52с and A6M7 Mod.63 was made on an individual basis depending solely on the available parts. The last attempt at Zero modernization was made in April of 1945. A6M5c was fitted with Mitsubishi MK8P Kinsey-62 engine, offering 1560 hp for take-off. The May tests of the two prototypes of A6M8 Model 54-hei finally showed significant increase in speed and climb rate, but none of the ordered 3600 machines were produced as the war ended. As for the two A6M8 prototypes, there is no record of their combat career.

Take off for the first Kamikaze attack from Mabarakat airfield in the Phillippines, 10.25.1944. A6M2 # 02-888 with a 250 kg bomb. A6M5 in the back on the right.	Zero kamikaze attack on Missouri near Okinawa, 04.11.1945
A6M7 on a captured Japanese airdrome, Fall 1945	Unsuccessful attack of A6M5 on light cruiser Columbia in Lingayen Bay, 01.06.1945