To obtain experience with jet propulsion a Saab team led by Ragnar Härdmark began studying the adaption of the propeller-driven Saab J 21A-1 for the 3,000 lb (1,361 kg) st de Havilland Goblin II jet engine. Four J 21A-1 airframes were assigned for conversion, redesignated J 21R (one of them is pictured), about 50% of the original airframe could be maintained instead of the predicted 80%.

The first J 21R fighter flew March 10, 1947 and the performance proved to be a little disappointing; the flying time was limited to approximately 45 min. The initially order for 120 aircraft was subsequently reduced to 60, 30 J 21RAs and 30 J 21RBs, the latter were powered by Swedish-built 3,300 lb (1,497 kg) st SFA Goblin II jet engines. The first J 21RA entered service early in 1950, soon being transferred to the ground-attack role and the last aircraft was delivered in 1952, while the type was withdrawn from service in 1956.