The Ki-21 was born of a 1936 Army specification that demanded long range and a good bomb load, and it delivered both — at a price. Japanese doctrine of the day assumed that fast, high-flying bombers could largely outrun interception, so designers traded protection for performance: defensive armament was light, fuel tanks were unprotected and there was little armour for the crew. That gamble worked spectacularly in China and in the opening Pacific offensives, when Allied fighter defences were thin or obsolete.

Once the Allies fielded fast, heavily-armed fighters and organised radar-directed defences, the same design choices turned deadly. “Sally” formations that had roamed almost freely in 1942 were being cut apart by 1943–44. Mitsubishi’s upgrades — the 1,500 hp Ha-101 engines, a heavier dorsal turret, extra guns — improved the aircraft but could not change its fundamentals. The Ki-21 is a textbook case of a competitive early-war design overtaken by the brutal pace of wartime fighter development.

The Ki-21-I entered service on two Nakajima Ha-5 Kai radials rated near 950 hp for take-off — adequate for the loads and speeds of 1938. As Allied fighters closed the gap, the type needed more power. The Ki-21-II re-engined the airframe with the Mitsubishi Ha-101 (Army Type 100), a 14-cylinder air-cooled radial of about 1,500 hp at take-off. The change raised maximum speed to roughly 486 km/h at altitude and improved climb and ceiling, but the extra weight and drag of later defensive additions ate into the gains. By the standards of 1944 the “Sally” was still under-powered for the threat it faced.

Early Ki-21s carried a scattering of hand-aimed 7.7 mm machine guns in the nose, dorsal, ventral, tail and beam positions — enough to worry a lone biplane, far too little against modern monoplane fighters attacking in numbers. Worse, the aircraft had unprotected fuel tanks. A few incendiary rounds in the wing could turn a “Sally” into a torch. The Ki-21-IIb’s dorsal turret and heavier 12.7 mm gun helped, but the fundamental vulnerability — no self-sealing tanks, minimal armour — was never fully cured. It is the single biggest reason the type’s loss rate climbed so steeply from 1943.

The Ki-21 was a wartime product of the Imperial Japanese state, built for the Army and never sold on any open market. As a result no credible flyaway unit price and no cost-per-flight-hour figure exist in the public record — not in period Japanese documents that survive in translation, nor in the standard post-war references. Any specific dollar or yen figure attached to a Ki-21 should be treated as guesswork. What can be said is comparative: the “Sally” was a mainstream twin-engine bomber produced in the thousands, so its unit cost sat well below Japan’s larger four-engine and naval types but above single-engine fighters of the same era.