Work began at SAAB following a Swedish Air Force decision to embark on a major expansion programme in preparation for the possibility of being drawn into the Second World War.

The company designed a monoplane twin-boom aircraft, powered by a single Daimler-Benz DB 605B engine that was positioned at the rear of the fuselage nacelle, driving a pusher propeller.

This unorthodox design possessed several advantages, such as the ability to concentrate most of the guns in the aircraft's nose, good pilot visibility, and ease of service.

As an emergency measure, the Swedish Air Force embarked upon a major rearmament and expansion of their military during the 1939–1941 period, which included the procurement of foreign-sourced aircraft as well as the local development of new, modern designs.

As a consequence, Sweden was forced to purchase already obsolete Fiat CR.42 biplanes from Italy as an interim measure, and which were of little value against modern monoplane fighters.

[5] The SAAB 21 was an unorthodox twin-boom low wing pusher configuration fighter aircraft with a tricycle landing gear, and a heavy forward-firing armament.

[6] Several recent innovations were incorporated into its design, including an ejection seat for the pilot while the pusher layout later allowed the type to be readily re-engined with a turbojet.

Many solutions were examined, such as jettisoning either the propeller or the engine via explosive charges prior to bailing out, before it was decided to adopt an ejection seat developed by Swedish defense firm Bofors, in parallel with the fighter.

[6] To reduce drag, coolers and ducts for the engine were located in the wing section between the fuselage and tail booms, and integral fuel tanks were fitted.

These improved firing performance but the Saab 21 was now only viable as a ground attack aircraft due to the rate of change in fighter design.

[10] During the first half of the year, the company was working on a variant that would be powered by a 1,500 kW (2,000 hp) Rolls-Royce Griffon engine to give the 21 a projected top speed of 669 km/h (416 mph).

However, within less than four years, some squadrons were already being re-equipped with a new generation of jet fighters capable of far greater speed, such as the de Havilland Vampire.

[10] This aircraft used the J 21's DB 605B engine and had a more traditional appearance, similar to the North American P-51 Mustang, but its projected performance was reduced, which became a key factor in improving the J 21 instead.

[7][16] J 21A-3 (later designated A 21A) Three aircraft are preserved on permanent static display; Data from Beskrivning över fpl typ 21A, häfte 1 (description of airplane type 21A, booklet 1),[19] Beskrivning över fpl typ 21A, häfte 6 kap L. Beväpning (description of airplane type 21A, booklet 6, chapter L. armament),[20] SAAB J21/J21R[21]General characteristics Performance Armament