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Mitsubishi was the first and largest series producer of Japanese engines during World War II, with a 1937 Pratt & Whitney arrangement possibly contributing to this success. This 18-cylinder, two-row, air cooled aircraft engine incorporated two-speed reduction gearing, a cooling fan, and twin transverse superchargers to achieve two stage supercharging. Four of these engines were to power the Kawasaki Ki-91 Army Experimental Long-range Bomber, which was to be larger than the Boeing B-29 and have a 4,474 km (2,796 miles) radius of action. The design of this bomber began in May 1943, and a prototype was under construction at the end of 1944, but planned production was suspended in February 1945 when tooling was destroyed during a B-29 raid before the prototype was completed. Between 1940 and 1945, a total of 2,813 Mitsubishi Ha 42 series (all models) were built.