The first A6M5 "Zero" was build by the Mitsubishi Jukogyo in 1943. It was developed to combat heavier built & armed Allied fighters that were knocking the earlier "Zero" 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" problems.