A quad-engined all-metal parasol-wing monoplane flying boat. An experimental version, the Type S, made its first flight on July 14, 1936. The plane was approved by the Imperial Japanese Navy as Type 97 Large Flying Boat in 1937. Mass production began in 1938.
The most common variant was model 22 (H6K4), created in 1940. The first 119 planes were fitted with a air-cooled 14-cylinder twin-row radial Mitsubishi Kinsei 43 engine (1000 hp). Afterwards, the slightly more powerful Kinsei 46 was used. The H6K4 carried more fuel than earlier models, increasing its maximum range to 25 hours, or 6000 kilometers. Its fuselage included waist blisters fitted with machine guns, which required a slightly longer tail section.
The H6K4 was armed with four 7.7mm Type 97 (Lewis) machine guns with 582 rounds apiece and, in the tail turret, one 20mm Type 99-I cannon with 432 rounds. The flying boat could carry twelve 60-kg bombs, four 250-kg bombs, or two 800-kg Type 91 Model 1 torpedoes.
A major weakness of all H6K flying boats was the lack of protective armor for the crew and for the fuel tanks. This was sacrificed for the sake of its extended flight range. When the plane started to suffer losses in combat, designers of later Model 22 planes began to focus on protecting the crew.
The H6K flying boat's first combat action was a raid on Wake Island on December 11, 1941. The plane was widely used in the Pacific theater during the first part of the war. The H6K ran long-range reconnaissance missions, convoy escort duty, submarine searches, bombing missions, and search and rescue operations. When the newer H8K arrived, the H6K began to be used mostly as a transport plane. The plane was in operation until the Japanese surrender in 1945.
127 H6K4s were built, out of a total of 217 H6Ks.