The Swiss Federal Aircraft Factory developed a design for a four-engined swept winged fighter following the end of the Second World War.

The bypass air could be deflected though large slots on the upper and lower wings to act as aerodynamic flaps or thrust reversers.

[2] The Armstrong Siddeley Mamba turboprop was then chosen as the basis for the N-20's prototype engines, with the propeller reduction gear replaced by a low pressure compressor.

[3] This powerplant was named Swiss Mamba SM-1; the conversion was carried out in 1948 by EFW, with only six engines built (four fitted to the N-20 with two spares).

[1] The aircraft was equipped with a JATO solid rocket motor, which was able to be started independently to reach the necessary altitude for tests.

[5] The full scale aircraft was estimated to have a maximum speed of 1,095 km/h (680 mph),[1] but the initial converted Mamba, the SM-1, which was test-flown under a de Havilland Mosquito in 1948 and was the first turbofan to fly,[citation needed] did not generate adequate thrust.

The aircraft was similar to the N-20.10, but had one conventional engine, a Rolls-Royce Avon or Armstrong Siddeley Sapphire turbojet in each wing root.