The Nakajima B5N prototype, designated B5N1 (in Allied reported name: Kate), was flown in January 1937. The first serial aircraft carried the designation Model 98 Model 11. The machines were tested in combat in China. In December 1939, another serial version entered the production marked as B5N2 Model 12. The aircraft was equipped with a Nakajima Sakae engine with a smaller frontal diameter and increased power. This improved plane's performance. Initially, a hydraulic wing tip folding mechanism was used, but after a short time it was replaced with a manual one, which was more reliable.