Kenichi Mitsumara's design was inspired by the Douglas DC-2 that Nakajima was building at the time under licence; a conventional, low-wing cantilevermonoplane powered by twin engines. Construction was of metal throughout, and the main units of the tailwheelundercarriage retracted into the engine nacelles. The bomb load was carried in an internal bay.
The prototype LB-2 was completed in March 1936.[2] It was considered for production, along with the Mitsubishi G1M, but eventually both were rejected in favour of the Mitsubishi Ka-15, which would be produced as the G3M. Adapted for civil use, the LB-2 prototype's bomb bay was replaced with a fuel tank and a cabin for six passengers was fitted, with the bombardier's position in the nose converted to store luggage.[2]