The first A6M5 'Zero ½ was build by the Mitsubishi Jukogyo in 1943. It was developed to combat heavier built armed Allied fighters that were knocking the earlier 'Zero ½ out of the sky during 1943. The Nakajima NK1F Sakae 21 engine gave the plane slightly better performance, and the armament was changed to 2- 20mm and 2- 13.2mm guns. These changes did not make up for the lack of armor, or the lack of self-sealing fuel tanks. However,despite these shortcomings, over 10,450 were built, being used during the entire war because other models of Japanese aircraft continued to have 'teething ½ problems.