Initial A6M5 �Zero� was build by the Mitsubishi Jukogyo in 1943. It had been developed to fight heavier built & armed Allied fighters that have been knocking the sooner �Zero� out of the sky during 1943. The Nakajima NK1F Sakae 21 engine offered the airplane notably better performance, therefore the armament was indeed changed to 2- 20mm and 2- 13.2mm weapons. These modifications don't change having less armor, or the insufficient self-sealing fuel tanks. But,despite these shortcomings, over 10,450 was in fact built, used through the whole war because other types of Japanese aircraft proceeded to possess �teething� problems.