In 1931, the Hiro Arsenal began the design of a modern twin- engined monoplane flying-boat as a replacement for its biplane Type 15 and Type 89 Flying-boats. The new aircraft was a scaled-down version of the Type 90-1 Flying-boat, with more emphasis on a practical design for an aircraft of this type. Chief designer for the project was Lieut-Cdr (Ordnance) Jun Okamura.
Originally, this all-metal aeroplane was powered by two 500hp water-cooled Type 91-1 or 600hp Type 91-2 engines mounted on struts well above the wing. After repeated tests and modifications over a long period, they were replaced by a set of 760hp Myojo engines. These Myojos were licence-built Pratt & Whitney Hornets of US design. Other changes in an effort to improve the design involved moving the shoulder-mounted wing to the top of the hull and repositioning the empennage. There were other subtle changes. Nearly each prototype differed from the previous one in some way in efforts to improve upon the design. This consumed much time, and, by 1937, further efforts to improve the design and production were terminated because the design was becoming obsolete.
The many variations in these flying-boats were responsible for two distinct designations for this type. Beginning in July 1933, the earliest configuration of the series to be accepted by the Navy was the Type 91-1 Flying-boat with the short designation H4Hl. These had water-cooled engines. After such changes as repositioning the two hull steps, replacing two-bladed propellers with four-bladed types, and changes in the tail configuration, a redesignation for the type was in order. More advanced models with the air-cooled Myojo engines and three-bladed propellers became the Type 91-2 Flying-boat (H4H2). Both models varied, however, some had shoulder-mounted wings and others were high-wing monoplanes. They also differed within each type designation by having either a straight or tapered trailing edge, and the later versions had the Junkers double-wing type flaps. The design was never cured of instability while on the water or of its poor ability to cope with waves. Also, general performance never reached projected figures for the design; however, lessons learned helped in the development of the Kusho Type 99 Flying-boat, H5Y1, that carried the Allied code-name Cherry during the Pacific War.
Production of the H4H series was undertaken by Kawanishi as well, even though experimentation on design changes continued at Hirosho. This caused uncertainties and slowed production. Within the Kawanishi Company, the flying-boat was known as the Type L, the first of which made its maiden flight on 16 June, 1933.
During the entire Sino-Japanese Conflict, these flying-boats were very active although used in small numbers. They patrolled along the coast of China, and served as transports for mail and cargo between the home islands of japan and across the East China Sea to the mainland. This brought about the claim that the Type 91 Flying-boats were the first Japanese flying-boats to be used in a war zone.
Twin-engined monoplane flying-boat with two-step hull. All-metal stressed skin construction with wing having a Wagner/Rohrbach box-spar. Crew of six to eight. Two 600hp Type 91-2 twelve-cylinder W-type water-cooled engines, driving four-bladed wooden propellers (H4H I), two 760hp Myojo 1 or 2 nine-cylinder air-cooled radial engines (H4H2). One bow-mounted flexible 7. 7mm machine-gun, twin dorsal flexible 7.7mm machine-guns. Bomb load: Two 250kg (551Ib) bombs.
Dimensions, weights and performance from Japanese Navy Technical Orders. Hiro built about thirty from 1932, Kawanishi built five in 1933, four in J934 one in 1935, four in 1936 and three in 1937. Approximate total forty-seven.