In 1941, Sweden started a domestic rearmament program in order to bolster the strength of its armed forces in an effort to ensure its neutrality during the ongoing war. Among the branches intended for reinforcement was the Swedish Air Force, which up until this point had mainly been operating foreign, partially outdated aircraft.

The SAAB company had already been working on a design study for a modern fighter aircraft since the early 1940s. Their project had a rather unorthodox design - a single-engine, twin-boom aircraft in a pusher configuration. With the arrival of the 1941 rearmament program, sparking a great interest for domestic aircraft production, SAAB began working on refining their design for mass production.

After further work, three full prototypes were constructed for testing, two of which were fully functional. In July 1943, the prototype of the SAAB 21 first took to the skies for its maiden flight. Production of the SAAB 21 began some time after its maiden flight, with the first aircraft reaching their units in December 1945.

Overall, over 50 J21A-1s were produced before production of the first model stopped by December 1946. In total however, close to 300 piston-engined J21s of all variants were produced before the production run of the type ended in 1949.

Due to the J21’s introduction to service coinciding with the dawn of the jet age, the type didn’t remain in active service for too long. In fact, the J21 was retired in 1954 - some ten years after its initial introduction - and its only operator remains the Swedish Air Force.