The Nakajima B5N Allied reporting name Katherine "Kate" was the sole shipborne torpedo bomber of the Japanese Navy at the start of the Pacific War. It was by then quite old, having been designed to meet a specification of 1935, and was already judged to be obsolescent. However, when first put into production it had been a very advanced aircraft, and in war it out performed any Allied carrier based torpedo plane until the arrival of the Grumman Avenger in mid 1942. In particular, it was greatly superior to the Douglas TBD Devastator the shipboard torpedo plane of the US Fleet at the crucial battles of Coral Sea and Midway. Concieved in 1936, the prototype of the 'Kate' first flew in Jan. 1937. It was intended for both torpedo and level bombing, in the latter role the bombardier viewing the target through the opening of small folding doors under the fuselage. The early B5N1 aircraft flew bombing missions over China. Most of them were converted to B5N1-K trainers. The definitive B5N2, which first flew in Dec. 1939, equipped carrier squadrons from 1939-1940.

The low-wing monoplane had a clean configuration with a fully retractable undercarriage. The large wings folded upward for carrier stowage and were arranged so that the wing tips overlapped the fuselage. The plane had a long, "greenhouse"-style cockpit and the radial engine had a variable-pitch propeller. The plane lacked armor protection for the crew and self sealing fuel tanks, hence it was highly vulnerable to allied fighters. Late in the war some were fitted with surface search radar or magnetic anomaly detectors for (land based) antisubmarine operations.

B5Ns played the main role in sinking the carrier Lexington at Coral Sea, Yorktown at Midway, and Hornet at the Battle of Santa Cruz in October 1942. Along with the destruction of the carrier Wasp by a Japanese submarine during the Guadalcanal campaign these were the major blows to the American carrier forces in the early stages of the War. These exploits supplemented the Kate's success in the surprise attack on Pearl Harbor, December 7 1941, in which 40 B5N2s armed with torpedoes and 103 B5N1s armed with bombs crippled the US Battle Fleet.

By the time of the Marianas campaign it had been largely replaced by its successor the Nakajima B6N Tenzan "Jill" but at the Battle of the Philippine Sea there were still 17 Kates in Admiral Ozawa's Mobile Fleet, aboard the light carriers of Division Three.The B5N2 had a maximum speed of 235 mph with a maximum range of 600 nm. The aircraft could carry a 1,764 lb torpedo or three 550 lb bombs. A single flexible 7.7 mm mahcine gun was fitted in the aft cockpit. It was flown by a three-man crew. Total production of the B5N was 1,149.