The Supermarine B.12/36 was a heavy bomber project developed in response to Air Ministry specification B.12/36, issued in July 1936. Designed as a high-speed, four-engine aircraft, it was intended to serve the Royal Air Force (RAF) with a versatile mission profile, capable of delivering a 14,000 lb (6,400 kg) bomb load or transporting up to 24 soldiers. The design was highly ambitious for the mid-1930s, with projected performance estimates including a maximum speed of 360 mph (580 km/h) at 14,000 ft, a cruising speed of approximately 260 mph, and an operating ceiling of 30,000 ft. Its estimated range was roughly 3,000 miles (4,800 km).
Supermarine, the manufacturer famous for the Spitfire, was ordered to build two prototypes under this specification, with the project linked to internal design numbers Type 316, 317, and 318. The aircraft was powered by four Bristol Hercules HE.1.SM air-cooled, two-row radial piston engines, each producing 1,315 hp. The projected airframe featured a wingspan of 97 ft 0 in, a length of 73 ft 6 in, and a wing area of 1,358 sq ft, resulting in an approximate gross weight of 55,000 lb. For defense, the bomber was designed to carry eight 0.303 in (7.7 mm) machine guns and could be configured to carry up to 29 bombs of 250 lb or 500 lb weights.
Despite the advanced design, the B.12/36 never entered production or operational service. The two prototypes were under construction at the Supermarine works at Woolston, Southampton, located on the River Itchen. On 26 September 1940, a Luftwaffe daylight raid bombed the Woolston facility, destroying both prototypes and the construction plans. Consequently, the Air Ministry formally cancelled the order in November 1940. The failure of the Supermarine project left the Short S.29 (which became the Short Stirling) as the primary result of the B.12/36 competition. Because no airframe was completed and all documentation was lost in the bombing, the B.12/36 remains a "paper aircraft" with no surviving examples or museum pieces.