In early 1943, following the cancellation of the Nakajima Ki-68 and Kawanishi Ki-85 due to the failure of the Nakajima G5N, Kawasaki responded to the Imperial Japanese Army's requirement for a long-range bomber of similar size and performance to the B-29 Superfortress with the Ki-91.

In April 1944, a wooden mockup was built for inspection and, by May, production of the first prototype was ordered.

Construction of the prototype began in June, but the first B-29 raids on Japan were underway beginning that month.

[1] The Ki-91 was a design for a heavy bomber similar in size and bombload to the B-29 Superfortress and Consolidated B-32.

It had a bigger wingspan and fuselage than the B-29 and B-32 and featured a pressurized cabin to allow high-altitude flights.