The Mitsubishi Ki-20 Army Type 92 Heavy Bomber was based on the Junkers G 38 passenger aircraft, and was an unsuccessful design that never saw combat.
The aircraft was produced in response to a Japanese Army order for a very large long range bomber. This aircraft was for use either against the British base at Singapore or the American fortifications at Corregidor Island, both targets a significant distance from the nearest Japanese base.
The Ki-20 was based on the Junkers G 38, a very large four engined passenger aircraft. Although Germany was forbidden to build her own military aircraft, German companies were allowed to modify existing civil aircraft to military roles for the export market. Junkers added military systems to the G 38 and gave it the new designation K 51.
Although the Ki-20 itself wasn't a great success, it did Mitsubishi a great deal of valuable experience. Nobushiro Nakata, the chief designer, and Kyonosuke Ohki went to Germany in 1928 to prepare for production in Japan, and they were followed later in the year by two engineers who examined Junkers manufacturing methods. In 1930 Keisuke Ohtsuka was sent to German to purchase the required tools, machiness and engineers, and in return Junkers sent a team to Japan.
The Ki-20 was a massive aircraft. It had a wingspan one metre than the B-29 Superfortress, and twice the wing area of the much later American aircraft. The B-29 had more than three times the engine power, and was nearly three times heavier, but was also three times faster.
The Ki-20 used the typical Junkers corrugated stressed skin (as seen on the Ju 52). It had a fixed undercarriage and a biplane tail. The massive wings were almost bat-like in shape, with a tapered trailing edge. The engines were carried in four slim nacelles. Unusually the aircraft's four gun turrets were carried on the wings. The upper turrets, each armed with two machine guns, were mounted at the rear of the outer engine nacelles.
Only six Ki-20s were built. The first was completed in 1931, using German parts assembled in Japan. The second aircraft also used German parts but after that an increasingly proportion of the components were produced in Japan.
The designation Ki-20 wasn't given to the aircraft until late in its existence, thus explaining the comparatively high number for such an early aircraft. When it entered limited service it was known at the Army Type 92 Heavy Bomber (some sources say Super Heavy Bomber).
Tests showed that the Ki-20 was too large, underpowered and unreliable. The six aircraft were used for research at bases in Japan and Manchuria. Work on a replacement began in 1936 when the Army issued new specifications that resulted in the Mitsubishi Ki-21.
The Ki-20 was kept secret for most of its existence. Its first public appearance came in January 1940 when three took part in a flypast in Tokyo. Soon after this the type was withdrawn from service, but their massive size meant that they were still impressive and they were exhibited around Japan.
Engine: Four 800hp Junkers L 88 twelve cylinder v liquid cooled engines or four 750hp Type Ju (Jumo 204) twelve cylinder liquid cooled diesel engine
Crew: 10 (captain, two pilots, bombardier/ nose gunner, flight engineer/ top gunner, four wing gunners)
Span: 144ft 4.25in
Length: 76ft 1.5in
Height: 22ft 11.75in
Empty weight: 32,875lb
Loaded weight: 56,103lb
Max speed: 125mph
Armament: Twin 7.7mm machine guns in nose and each upper wing turret, one 7.7mm machine gun in each lower wing turret, on 20mm cannon on upper fuselage
Bomb load: 4,409lb normal, 11,023lb maximum