The Boulton Paul P.121 was a design for a supersonic fighter that was produced with fixed or variable sweep wings.
The P.121 was produced in response to Air Ministry specification ER.110T. Boulton Paul submitted two closely related designs, both with the designation P.121. Both designs had a long, wide flat fuselage, slightly wider at the top than the bottom. The aircraft would have been powered by two Rolls-Royce RA.8 engines, with a split air intake in the flat nose, the main engines deep in the fuselage, and the exhaust pip below and slightly ahead of the tail. The tail itself was sharply swept back, with the horizontal surface low mounted on the tail. The P.121 would have been armed with one 30mm cannon, mounted below the cockpit. It would have had an arrester hock to allow for carrier operations.
The two versions differed in their main wings. One had a fixed but very highly swept back wing. The other had a variable sweep wing, which would be moved back for high speed flight or forwards to offer more lift at slow speeds and at take off or landing. The P.121 didn't get past the basic design stage.