SAAB wanted to gain experience with jet engines, and decided in 1945 to convert three J 21A-1s to jet power with de Havilland Goblin 2 engines. The aircraft to be converted were taken from the assembly line before they were finished in November 1945. Optimistically it was thought only 20% needed to be redesigned, but closer to 50% had to be changed. First prototype flew on the 10/3/1947. The main differences was that the tailplane had to be moved up to get away from the exhaust and the wing leading edge moved forward and made sharper(a totally new wing was canceled on cost grounds). The RSAF planned to order 120 but only 60 were ordered in 1947
The J 21R wasn't a success as a fighter. So it was used as a ground attack aircraft but had very limited range, with a maximum endurance of 46 min. With wing tip tanks it was increased to 100 min, but loaded with rockets flying at low altitude the radius of action was only 190 km.