In 1937, the Imperial Navy of Japan formulated the technical task of the 12-Ci for the construction of a forward three- or four-engined flying boat.
Such a large range of flight was due to the desire of the fleet to cause torpedo-bombing strikes on the ships of the US Navy in Pearl Harbor without intermediate refueling at sea.
The designers attached great importance to the aerodynamic characteristics of the aircraft, so the fuselage, wing and engine bonnet had a minimum number of protruding parts.
Attempts by the designers to reinforce the structure of the glider led to an increase in mass, which made it impossible to reach the Hawaiian Islands without refueling.
And with refueling in the sea from submarines strike at Pearl Harbor could design a flying boat Kawanishi H8K, which could carry a significant bomb load and was well protected.