The Nakajima B5N1 first flew in January 1937. It was quite unique for a carrier-aircraft with its low mono wing that folded for storage and wide-track inward folding retractable landing gear. The new Kates were far superior to its competition, the TBD Devastator and the Fairey Swordfish however it did have its shortcomings. It was very poorly armed for self defense and required A5M escort protection. Also the crew and fuel tanks had no protection. The B5N1 was replaced by the improved B5N2.
In the first wave attack on Pearl Harbor on December 7, 1941 the group leader of the Japanese aircraft carrier Kaga was Lt. Ichiro Kitajima flying his Nakajima B5N2 torpedo bomber AII-3II. In all 40 B5N1 Kates and 49 B5N2 Kates took part in the initial raid with 54 more B5N2s arriving in the second wave along with 26 Aichi D3A Val dive bombers. When it was over Japan had sunk the USS Arizona, USS Oklahoma, USS West Virginia and USS California, USS Nevada sinking.