The Kate’s advantage in 1941 was not any single feature but the way it combined them. Where the US Navy still flew the Douglas TBD Devastator — a monoplane, but slow and short-ranged — and the Royal Navy relied on the Fairey Swordfish biplane, the B5N brought together a retractable undercarriage, an enclosed cockpit, Fowler flaps, a constant-speed propeller and integral (initially unprotected) fuel tanks in a clean, aerodynamically efficient airframe. Its folding wings let carriers pack more aircraft aboard.

The trade-off was deliberate. Japanese naval doctrine prized range, striking power and pilot skill over survivability, so the Kate carried no armour for its crew and no self-sealing fuel tanks, and only one rear-firing 7.7 mm gun for defence. Against 1941-vintage opposition this was an acceptable gamble; as the war turned and Allied fighters such as the F4F Wildcat and later the F6F Hellcat multiplied, the same design choices made the Kate dangerously exposed.

The armour-piercing bomb the Kate carried over Pearl Harbor was not a purpose-built weapon but an improvisation of genius and desperation: the Type 99 No. 80 Mark 5, fashioned from an obsolete 40 cm (roughly 16-inch) naval gun shell fitted with fins. Released from high altitude, it could punch through a battleship’s armoured decks before detonating deep inside. At about 08:06 on 7 December 1941, one such bomb is generally credited with reaching a forward magazine aboard USS Arizona; the resulting explosion tore the ship apart and killed more than a thousand of her crew. Precisely which aircraft dropped it, and the exact mechanism, remain debated, but the weapon and the aircraft that delivered it are not in doubt.

The qualities that made the Kate so effective in 1941 — light weight, long range, a heavy warload — were bought by leaving out protection. The aircraft carried a single 7.7 mm machine gun for defence, no armour plate for its three-man crew and, initially, no self-sealing fuel tanks. A slow, straight torpedo run at low level made it an easy mark once opposing fighters got among the formation. At Midway in June 1942 and again at Santa Cruz that October, Kate squadrons pressed home their attacks at heavy cost. Japan’s intended successor, the B6N Tenzan, was faster and better armed, but arrived slowly, so the ageing Kate soldiered on in front-line and second-line duties well into the war.

The B5N was a wartime product of the Japanese state, built in the late 1930s and early 1940s by Nakajima and, later, by Aichi and the Dai-Juichi naval arsenal. It was never sold on any open market, and Japan’s wartime accounting was not published in a form that yields a clean flyaway unit price or a cost-per-flight-hour figure. Any precise dollar or yen cost attached to a Kate today is, in practice, an estimate or a conversion of fragmentary records rather than a sourced procurement price. What is well established is the production total — roughly 1,149 aircraft across all builders.