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.
Ki-21-I - Prototype,
powered by the Nakajima Ha-5 engine, demonstrated performance equal to
any of the world's contemporary bombers in the same category.
Ki-21-Ia - It was
ordered into production as the Army Type 97 Heavy Bomber Model 1A
(Mitsubishi Ki-21-la), enter ing service in the summer of 1938.
Operational experience in China showed the type to be deficient in
armament, leading to the improved Ki-21-lb.
Ki-21-Ib - With five
instead of three machine-guns and an enlarged bomb bay.
Ki-21-Ic - With
increased fuel capacity and one extra machine-gun.
Ki-21-II - Continuing
development brought four Ki-21-11 prototypes, introducing more powerful
Mitsubishi Ha-101 engines.
Ki-21-IIa - With the
same armament as the Ki-21-lc this model (Ki-21-II) entered production
as the Ki-21-lla.
Ki-21-IIb - Final
production version was the generally similar Ki-21-llb which
incorporated some refinements.
MC-20-I and Army Type
100 Transport Model 1 (Ki-57 "Topsy") - In addition to the military
Ki-21s, about 100 Ki-21-la aircraft were later converted for use as
unarmed civil freighter/transports.
MC-20-II and Army Type
100 Transport Model 2 (Ki-57 "Topsy") - An improved model with
Mitsubishi Ha-102 radials in redesigned nacelles. Also incorporated a
number of detail refinements and equipment changes. About 406 aircraft
were built before production ended in January 1945.
L4M1 (Ki-57 "Topsy") -
A small number of MC-20-I aircraft transferred to the Japanese Navy.
(Army Type 97 Heavy
Bomber Model 1A - Mitsubishi Ki-21-IIb)
Type: Five or
Seven Seat Heavy Bomber
Mitsubishi Design Team
Mitsubishi Jukogyo KK, also built by Nakajima Kikoki KK
(Ki-21-I) Two 850 hp (634 kW) Nakajima Ha-5-Kai 14-cylinder two row
radial engines. (Ki-21-IIb) Two 1,500 hp (1119 kW) Mitsubishi Ha-101
14-cylinder radial engines.
Maximum speed 301 mph (485 km/h) at 15,485 ft (4720 m); service ceiling
32,810 ft (10000 m); initial rate of climb 1,640 ft (500 m) per minute.
miles (2700 km) on internal fuel.
13,382 lbs (6070 kg) with a maximum take-off weight of 23,391 lbs
73 ft 9 3/4 in (22.50 m); length 52 ft 6 in (16.00 m); height 15 ft 11
in (4.85 m); wing area 749.19 sq ft (69.60 sq m).
7.7 mm (0.303 in) Type 89 machine-guns (in nose, ventral, tail, and
port and starboard beam positions) and one 12.7 mm (0.50 in) Type 1
machine-gun in dorsal turret plus up to 2,205 lbs (1000 kg) of bombs.
Ki-21-I (prototype), Army Type 97 Heavy Bomber Model 1A (Mitsubishi
Ki-21-Ia), Ki-21-Ib, Ki-21-Ic, Ki-21-II, Ki-21-IIa, Ki-21-IIb, MC-20
(Ki-57) unarmed transport. (See Mitsubishi Ki-57 in transport section).
flight November 1936; service delivery 1937; first flight (Ki-21-II)
mid-1940; final delivery September 1944.
(Imperial Japanese Army), Thailand.