The Experimental Kusho 12-Shi Special Flying-boat H7Y1 was a highly secret attempt to produce a long range flying boat that could reach Hawaii from Japan and return safely with its photographs.
The Japanese Navy began work on the long-range flying boat in 1937. It was considered to be top-secret, and as a result the normal procedure of asking for designs from several firms was abandoned. Instead a single design was produced at the Kusho naval arsenal (better known as Yokosuka). The design team was led by Lt-Cdr (Engineering) Jun Okamura.
The new aircraft needed a maximum range of 5,000 nautical miles (5,750 statute miles), longer than any contemporary aircraft. Only the Dornier Do 26 was credited with a similar range, of 5,620 statute miles.
The Do 26 was a high-winged monoplane, with four 600hp Junkers Jumo engines mounted in pairs, one pusher and one puller on each nacelle. When the Japanese began work on the 12-Shi Special Flying-boat the Do 26 had yet to make its maiden flight, but its range did live up to expectations.
The Japanese Navy gave Kusho a fairly free hand. The new aircraft was to use two 510hp Junkers Jumo 205 diesel engines, have high-mounted wings and a crew of four. It was to have a slim hull and retractable wing-tip floats to reduce drag.
A single prototype of the H7Y1 was completed in 1939. The prototype underwent flight tests in which a series of problems were discovered. Determined efforts had been made to reduce the weight of the aircraft, and as a result the wing centre section and the tail both suffered from excessive vibrations. The tail vibration also spread along the fuselage, making the cabin uncomfortable. The aircraft was underpowered, and probably would have struggled to take off with a full fuel load.
Work began on improving the design, but the Japanese Navy then decided that it didn't need such a long range reconnaissance aircraft after all. The project was cancelled, and in order to prevent any hints of it from leaking the prototype, all photographs and all paperwork was destroyed.