The Kawanishi H6K was an Imperial Japanese Navy flying boat used during World War II for maritime patrol duties. The Navy designation was "Type 97 Large Flying Boat".
The aircraft was designed in response to a Navy requirement of 1934 for a long range flying boat and incorporated knowledge gleaned by a Kawanishi team that had visited the Short Brothers factory in the UK, at that time one of the world's leading producers of flying boats, and from building the Kawanishi H3K, a license-built, enlarged version of the Short Rangoon. The Type S, as Kawanishi called it, was a large, four-engine monoplane with twin tails, and a hull suspended beneath the parasol wing by a network of struts. Three prototypes were constructed, each one making gradual refinements to the machine's handling both in the water and in the air, and finally fitting more powerful engines. The first of these flew on 14 July 1936. Eventually, 217 would be built.
The first major production version, the H6K4 was powered by four Mitsubishsi Kinsei 43 radiais and armed with four 7.7mm machine-guns in bow and midships positions and a 20mm cannon in a tail turret, and was capable of carrying two 800kg bombs or torpedoes, a total of 66 being in service at the time of Pearl Harbor; later aircraft were powered by Kinsei 46 engines. These boats were widely employed, although the initial heavy defeats inflicted on the Allies in the Pacific rendered maritime reconnaissance duties subordinate to the need for air transportation of Japanese troops during the swift conquests in the East Indies and elsewhere.
A number of aircraft, designated H6K4-L, were converted for transport duties and were each able to accommodate about 18 fully-armed troops; lacking armour and self-sealing fuel tanks, however, they were extremely vulnerable to fighter attacks and, after a number had been shot down, a new version entered production as the H6K5 in August 1942; by that time the maritime reconnaissance version had been given the reporting codename 'Mavis' by the Allies, the transport derivative being named 'Tillie'.
Powered by either Kinsei 51 or 53 radials, the H6K5 was intended to eliminate the shortcomings of the earlier versions, but although the open bow gun position was replaced by a single-gun turret immediately aft of the pilot's cockpit, the overall armament was not increased. Only 36 H6K5s were completed by 1943, when production gave place to the greatly superior H8K.
H6Ks served with the 8th, 14th, 801st, Toko and Yokohama Kokutais, and some of the H6K5s were employed as naval staff transports throughout the Pacific in 1943. Eighteen aircraft served on the quasicommercial courier services in South East Asia, a number of them being destroyed by Allied aircraft both in the air and at their moorings.