The Supermarine Type 391 was the last piston-engined fighter to be proposed by Supermarine.^[1]

In 1943, as the design of the Spiteful and Seafang was proceeding, Supermarine presented a brochure to the Air Ministry describing a "High-Performance Aeroplane for the Royal Navy", designated internally as the Supermarine Type 391.^{[note 1]} Of greater length and span than the Seafang, the Type 391 was intended to be used as a carrier-borne fighter, with a secondary role as a strike aircraft.^[2]

The wing of the Type 391 was similar in outline to that of the Spiteful/Seafang but was larger and, unlike all previous Supermarine fighters, was mid-mounted. The whole section outboard of the undercarriage hinged up, while a wide centre-section incorporated thick roots with tapered leading-edge air intakes. These fed the engine cooling radiators. Because of the higher-mounted wing, the main undercarriage legs were also longer, with the wheels pivoting when raised to fit into wells in the fuselage.^[1] The forward fuselage was lengthened to accommodate the new engine, while the rear had redesigned and enlarged tail surfaces.^[1]

The Type 391 was eventually rejected in favour of the Type 392, which became the Supermarine Attacker.^[2]

Data from ^[3]

General characteristics

• Crew: 1
• Length: 39 ft 9 in (12.12 m)
• Wingspan: 43 ft 6 in (13.26 m)
• Wing area: 335 sq ft (31.1 m²)
• Gross weight: 17,250 lb (7,824 kg)
• Powerplant: 1 × Rolls-Royce Eagle H-24 liquid-cooled piston engine, 3,550 hp (2,650 kW)
• Propellers: 8-bladed contra-rotating constant-speed propeller

• Andrews, Charles Ferdinand and Eric B. Morgan. Supermarine Aircraft Since 1914. London: Putnam Aeronautical Books, 1981. ISBN 978-0851778006.
• Buttler, Tony. British Secret Projects: Fighters and Bombers 1935–1950. Hersham, Surrey, UK: Midland Publishing, 2004. ISBN 1-85780-179-2.
• Pegram, Ralph. Supermarine Secret Projects, Vol 2: Fighters and Bombers. Horncastle, Lincolnshire, UK: Mortons Media Group Limited, 2022. ISBN 9781911703044.