It saw service in the late 1940s and early 1950s before it was replaced by a new generation of fighters designed from the onset with jet propulsion, such as the de Havilland Vampire and the Saab 29 Tunnan.

During the early stages of the Second World War, Sweden was concerned that its neutrality and its independence could be threatened by one of the belligerent powers, and so enacted a series of emergency measures to increase its military's preparedness and deterrence value against potential aggressors.

Between 1939 and 1941, the Swedish Air Force committed itself to a major expansion programme, which included the procurement of large numbers of foreign and locally developed fighters.

[2] As a consequence of the conflict, few nations possessed available production capacity or a willingness to supply modern fighters to Sweden, being a relatively small neutral country.

[4] The company envisioned an unorthodox twin-boom pusher configuration fighter aircraft, featuring a low wing, a tricycle landing gear, and with a heavy forward-firing armament.

[5] It was powered by a licensed version of the new German Daimler-Benz DB 605B inline piston engine, which was refined and built by Svenska Flygmotor AB.

During the first half of the year, a variant was planned with a 2,000 hp (1,500 kW) Rolls-Royce Griffon engine which would have raised the top speed to 416 mph (669 km/h).

[8] Recognising the Swedish Air Force's enthusiasm for a jet-powered aircraft for late 1945, SAAB decided to produce a version of the 21 harnessing jet propulsion.

It was recognized that Sweden would otherwise be left behind while nations such as the United Kingdom already had the Gloster Meteor and de Havilland Vampire in production.

In all, in over 50 per cent of the airframe, tailplane and wing was changed and, each of them was equipped with a single British-sourced de Havilland Goblin turbojet engine, which replaced the DB 605B and propeller unit.

Data from Beskrivning över fpl typ 21R, häfte 1 (description of airplane type 21R, booklet 1),[16] Beskrivning över fpl typ 21R, häfte 6 kap L. Beväpning (description of airplane type 21R, booklet 6, chapter L. armament),[17] SAAB J21/J21R[18]General characteristics Performance Armament