The Ki-21 (Ki = Kitai = Airframe) was to the Japanese Army what the G4M 'Betty' was to the Japanese Navy: their best bomber. Both aircraft were built by , had an excellent performance and incredible range, but at the cost of structural strength, offensive warload, and armor protection.
The type was the result from a February 1936 requirement for a modern bomber to replace both the Mitsubishi Ki-20 (Army Type 92 Heavy Bomber) and the Mitsubishi Ki-1 (Army Type 93 Heavy Bomber). It was a clean monoplane of essentially all-metal construction with fabric-covered control surfaces, a oval-section fuselage with enclosed accommodation, a plain tail unit of cantilever construction, a cantilever mid-set dihedraled wing that was tapered in thickness and chord and carried inboard flaps and outboard ailerons it its trailing edges, and tailwheel landing gear that included main units that retracted into the underside of the nacelles for the two wing-mounted engines.
The first of eight prototype and service trials aircraft made its maiden flight in December 1936 with a powerplant of two Mitsubishi Ha-6 radial piston engines each rated at 825 hp (615 kW) and driving a three-blade metal propeller of the variable-pitch type, and the type was later ordered into production as the Ki-21-Ia or Ki-21-I Ko.