Why Did Britain Put a Tank Gun Inside an Aircraft

In 1943 the RAF took the Ordnance QF 6-pounder — the same 57mm gun that armed the turret of a Churchill tank — and built it into the nose of a de Havilland Mosquito made of plywood and balsa. The aircraft that carried it never attacked a tank. This is the full story of why the gun went to sea instead, what it was designed to kill, and why a weapon that worked was replaced by one that cost a fraction as much. The Mosquito FB Mk XVIII, nicknamed the "Tsetse", was Coastal Command's answer to a specific and expensive problem. Between August 1942 and January 1943 the Royal Navy's own post-war Staff History calculated that every U-boat sunk in the Bay of Biscay cost 12,350 flying hours and 34 aircraft. Attacking with depth charges meant flying to fifty feet and releasing within yards of a hull ringed with automatic guns. The 6-pounder offered a way out of that: solid armour-piercing shot that could pass through two feet of water and still hole a U-boat's pressure hull at 1,400 metres — a kill from nearly a mile away, outside the flak. Making it work meant solving the problem that had defeated every previous attempt since the Royal Naval Air Service fitted Davis recoilless guns to flying boats in the First World War.