Mitsubishi Ki-21 Sally or 'Type 97 Heavy Bomber - 九七式重爆撃機 in a nutshell

National origin:- Japan
Role:- Bomber, Transport	Variants:- Mitsubishi Ki-57
Manufacturer:- Mitsubishi Heavy Industries	Produced:- 2,064 units (excluding Ki-57)
First flight:- 18th December 1936	Retired:- 1945 (Japan)
Introduction:- 1938	Primary users:-    Japan,   Thai Air Force, post war    Indonesia

Mitsubishi Ki-21 Sally

Winning a production order in November 1937, following competition with Nakajima's Ki-19 prototype, the Mitsubishi Ki-21 was designed and built to meet an Imperial Japanese Army requirement of early 1936 for a four-seat bomber that would have a maximum speed of at least 249 mph (400 km/h) and an endurance of more than 5 hours. Few twin-engine bombers anywhere in the world could exceed such performance at that time and, not surprisingly, the Ki-21 was later recognised as the best bomber in Japanese service during World War II. A cantilever mid-wing monoplane of all-metal construction, the design incorporated retractable tailwheel landing gear, a ventral bomb bay and two radial engines, one mounted in a nacelle at the leading edge of each wing. As first flown, on 18 December 1936, the Ki-21 had 825 hp (615 kW) Mitsubishi Ha-6 radial engines, but competitive evaluation against the Nakajima Ki-19 powered by that company's Ha-5 engine led the army to instruct Mitsubishi to introduce similar engines on the Ki-21. When the aircraft had been tested again with revised vertical tail surfaces and these more powerful engines, the army had no hesitation in ordering the aircraft into production under the designation Army Type 97 Heavy Bomber Model 1A, company designation Mitsubishi Ki-21-Ia. The first of the production aircraft began to enter service in the summer of 1938 but, when used operationally in China later that year, they were soon found to be lacking in defensive armament and self-sealing fuel tanks.

Improved versions were developed to overcome these and other shortcomings, the Ki-21-Ib introducing revised horizontal tail surfaces, larger area trailing-edge flaps, an enlarged bomb bay and armament increased to a total of five 7.7 mm (0.303 in) machine-guns. The generally similar Ki-21-Ic differed by having increased fuel capacity and the addition of one more 7.7 mm (0.31 in) gun. To increase performance four improved Ki-21-Ics were given more powerful Mitsubishi Ha-101 engines and these, redesignated Ki-21-II, were used for service trials. Ordered into production as the Army Type 97 Heavy Bomber Model 2A (Mitsubishi Ki-21-IIa), this version was operated by most of the army's heavy bomber groups at the beginning of the Pacific war. These aircraft played a significant role in. the opening phase of the war, but as Allied resistance began to increase and bomber crews found themselves confronted by fighter aircraft of increased quality and in greater quantity, Ki-21 losses began to rise steeply. Further revisions of defensive armament were made, the Ki-21-IIb replacing the dorsal gun position by a manually operated gun turret containing one 12.7 mm (0.50 in) machine-gun, and this version also introduced redesigned cockpit canopies and individual engine exhaust stacks to give some thrust augmentation. However, it soon became clear that the Ki-21 was gradually becoming obsolescent, and during the last year of the war the maj6rity were relegated to second-line duties. Allocated the Allied codename 'Sally', the Ki-21 was built to a total of 2,064 by Mitsubishi (1,713) and Nakajima (351). From this total a number of Ki-21-la aircraft were modified to serve as freight transports for use by Greater Japan Air Lines. Designated MC-20, these aircraft had all armament and military equipment removed and could, if required, be fitted with nine troop seats.