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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 Mitsubishi, 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.

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.

The Mitsubishi Ki-21 (codenamed "Sally" by the Allies) was a successful medium bomber in service with the Japanese Air Force since 1939, seeing combat action up until 1945 despite the fact that the aircraft was already outclassed by the crop of American and English airplanes in the theater. Nevertheless, the Ki-21 proved a success since its inception and would become even more lethal when in use as kamikaze aircraft. While an adequate aircraft in its own right, the Ki-21 suffered form the same major drawbacks inherent in most of the Japanese designs of World War 2 - inadequate defensive armament and armor protection for the crew.

Externally, the Ki-21 was a clean design, sharing some layout similarities to its American counterparts the B-25 Mitchell and the B-26 Marauder. The system yielded two engines on a mid-mount monoplane wing assembly. The cockpit was fitted just forward of the wing root providing the pilot a decent view of the engines to either side of him. The nose featured a green-house type glass arrangement with more small windows positioned to look downward than upward. A dorsal turret was fitted just past the wings with a single vertical tail surface adorning the empennage. Armament on early models consisted of a few 7.7mm machine guns though this would be upgraded to several more in later models including a heavy caliber 12.7mm type in the later Ki-21-II model series. Single instances of guns were positioned in the nose, dorsal turret, a ventral position and two beam positions. Beyond that an internal bomb load capacity of 2,205 pounds was afforded. Crew accommodations amounted to five personnel.

The Ki-21 was initially drawn up by the Mitsubishi corporation to an Imperial Japanese Army Air Force requirement appearing in 1936 with prototypes airborne by the end of that year. Despite this quick development, the system would have to wait till te end of 1939 to be fielded en mass thanks to delays along the production front. By then, the Ki-21 was already being fielded in China and elsewhere after that. Initial combat reports signaled the deficiencies in the design - primarily with armor and armament - and, as such, the system appeared in a "b" and "c" variant class to address this. Despite this effort, the initial production line of Ki-21's was already wholly obsolete and a newer version known as the Ki-21-II appeared with Mitsubishi-brand 1,500 radial engines in place of the original Nakajima types. This new model series spawned into an "a" and "b" variant model which differed mostly in armament - the latter showcasing Type 1 12.7mm (.50 caliber) machine gun in a pedal-operated dorsal turret.

The Ki-21 found many an initial success in the early years of the war in the Pacific, thanks in part to a solid design and to the general unprepared nature of the Allies. By the end of the war, operation of the Ki-21 was limited to the macabre delivery of kamikaze units to which the potent internal payload was put to extensively lethal use. The Ki-21 was to be superceded in production by the newer Nakajima Ki-49 Donryu "Helen", though that aircraft in itself was a general disappointment.

When the wider Pacific War opened, Ki-21s were still the Army’s primary heavy bomber and were used in that role against the Philippines, Malaya and Burma as the Japanese offensives opened. Although Ki-21s had proved capable against Chinese fighters, the modern types in use with Allied forces such as the Curtiss P-40 proved more than a match. Improved models with more powerful engines and extra defensive guns did not completely solve the problems with the bomber. Despite this, the Ki-21 remained in service until the end of the war, despite the arrival of replacements such as the Ki-49 and Ki-67 heavy bombers.

The Ki-21 was assigned the Allied Reporting Name “Sally” in 1942. Later the Ki-21-IIb was mis-identified as a new type and assigned the codename “Gwen” before the mistake was rectified and the new type was re-designated the “Sally III”. There is also an apocryphal story that the Ki-21 was codenamed “Jane”, after General MacArthur’s wife, before he objected and “Sally” was assigned instead. This story, however, does not make sense as the General’s wife was actually called Jean, and there appears to have been no objection to the codenaming of the B4Y Type 96 “Jean” torpedo bomber. Perhaps “Jane” was briefly assigned to the Ki-21-Ib (which had a re-designed tail and extra defensive machine guns) or the Ki-21-IIa (with more powerful engines) before it, too, was discovered to be a variant of the “Sally” - the “Sally II”. The codename “Doris” was also assigned to a non-existent “Mitsubishi B-97” bomber, which may have been confused with the Ki-21.