Sweden elected to remain neutral during the fighting of World War 2 (1939-1945) but this did not mean that it stood blind to the situation around it. The Soviet Union had invaded neighboring Finland and the Germans took Norway leaving Sweden as the lone Scandinavian power (Denmark too had fallen to the Germans on the European mainland). The Swedish Air Force drew up a request for a new modern fighter in 1941 intended to replace the now-obsolete American- and Italian-originated types that were still in frontline use for the service.
Design work on the aircraft followed and was led by Frid Wanstrom of Svenska Aeroplan AB. The team began with a single-seat, single-engine concept and this evolved to include a twin-boom layout. To provide for better forward views out-of-the-cockpit, the engine was set aft of the pilot in a "pusher" configuration. Not only was situational awareness a strong quality but the nose assembly was now free to be fitted with a collection of guns (as was the case with the American Lockheed P38 Lightning fighter). The pilot sat under a heavily-framed canopy and the raised fuselage spine restricted views to the rear. A straight monoplane wing planform was used that featured clipped tips and slight sweepback along the leading and trailing edges of the outboard planes. The empennage had a twin-finned configuration with a central plane joining the two fuselage booms. A modern tricycle undercarriage, fully retractable, rounded out the design elements which proved the aircraft rather innovative for its time.
The engine of choice became the German Daimler-Benz DB 605B 12-cylinder inverted Vee engine and this drove a three-bladed propeller unit at the rear of the cockpit nacelle, forcing air over and under the tail plane. Intakes were built into the wing leading edges (at the wing roots) to help aspirate the engine at rear. These powerplants would be built locally, under license, through SFA (Volvo Aero).
In terms of armament, the aircraft was given 1 x 20mm cannon along with 2 x 13.2mm machine guns and these were all fitted in the nose assembly. An additional 2 x 13.2mm machine guns were seen in the wings - one per wing.
A prototype made it to the air for the first time on July 30th, 1943 and two more followed for the test phase to come. The aircraft was given the designation of "J21" and Saab was charged with its serial production. The first operational-quality form became "J21A-1" and these served with the Flygvapnet through deliveries beginning in July of 1945. However, the war in Europe had ended in May of 1945 and the World War, in whole, would be over with the Japanese surrender of August. The Swedish Air Force took on a stock of 54 J21A-1 fighters nonetheless.
At the time of its adoption, the J21 was the only frontline fighter of pusher configuration to be adopted for service in World War 2. It was also the second fighter type anywhere in the world to feature an ejection seat as standard - this design element essentially forced upon engineers by the fact that the pilot would be vacating his doomed aircraft ahead of the spinning propeller blades - he simply could not roll off of the wing as usual and hope to clear the blades naturally. The German He 219 "Eagle-Owl" night-fighter became the first operational-level aircraft fitted with an ejection seat and the feature became standard on all fighters since.
In practice, the J21A-1 was not an outright success for her pilots found her heavy at the controls and the engine proved a temperamental beast prone to overheating and, in turn, reducing the expected performance. The J21A-2 was brought along to remedy some of the initial failings of the A-1 model and mainly concentrated on internal revisions. From this point forward, the J21A was no longer considered a viable frontline fighter by the Swedish air service and thusly relegated to the attack role in the "J21A-3" guise. 119 were delivered to this revised standard between the span of May 1947 and January 1949 with the key quality of weapon hardpoints being added for the carrying of drop bombs, fuel and rockets. A bomb-aiming sighting device was also fitted for accuracy and there was support for RATO (Rocket-Assisted Take-Off) canisters for quick take-off and climbing-to-altitude.
The J21A-3 featured a length of 10.45 meters, a wingspan of 11.5 meters and a height of 3.97 meters. Its listed empty weight was 7,165 pounds with its Maximum Take-Off Weight (MTOW) rated at 9,730 pounds. Power was from a SFA (Daimler-Benz) DB 605B 12-cylinder liquid-cooled inline piston engine developing 1,475 horsepower. Maximum speed reached 400 miles per hour with a cruise speed nearing 310 miles per hour. Range was out to 465 miles and the reported service ceiling was 36,100 feet. Rate-of-climb was 2,950 feet-per-minute.
All was not over for the J21 for, in 1945, further work was had on the series in an attempt to fit the line with a turbojet engine - the British de Havilland "Goblin" to be exact - and attempt to bring more out of the design than initially intended. Saab engineers took aside four J21 airframes for the test phase and the aircraft was accordingly modified to take on a turbojet powerplant with side-mounted intakes and a longer, deeper fuselage nacelle. The design stayed largely faithful to the prop-driven form with about half of the original design being retained, expediting development and keeping costs in check. The converted aircraft were designated as "J21R" and a prototype first took to the air on March 10th, 1947.The Swedish Air Force commission for about 120 of this new form.
The J21R entered service during early 1950. Its standard armament fit was the 1 x 20mm cannon with 4 x 13.2mm heavy machine guns. Additional provision was added for it to take on an 8 x 8mm machine gun pack or 8 x 14.5cm rockets for the ground attack role. Once in service, the J21R did not reflect much of an improvement for the line - the airframe and its configuration were essentially a technological dead end in terms of aerodynamics for the aircraft's maximum speed clashed with the critical Mach number and controlling was still a poor quality. Range also suffered through the thirsty turbojet engine which restricted flying time to just 40 minutes and all this led to the air service reducing their intended stock to half - sixty fighters - thirty of the lot then became "J21RA" forms fitting the de Havilland "Goblin II" turbojet of 3,000lb thrust and the remaining thirty being of the "J21RB" with "Goblin III" engines of 3,305lb thrust output. Again, the line was pushed into the attack role as its primary mission set and received the new designation of "A21R". These aircraft served until July 1954 before being given up for good.
The J21R model exhibited a running length of 10.45 meters with a wingspan of 11.37 meters. Its empty weight was 7,055 pounds and a Maximum Take-Off Weight (MTOW) of 11,025 pounds was listed. Power from the Goblin II engine produced 3,100 pounds thrust providing a maximum speed of 500 miles per hour with a range of 450 miles and a service ceiling up to 39,400 feet.
Around 300 Saab J21 aircraft were completed. The J21B was a proposed form intended to carry a battery of 3 x 20mm cannons in the nose as well as radar within its starboard side boom assembly. The model would have been powered by a Rolls-Royce Griffon or Daimler-Benz DB 605E propeller-driving engine but this program fell to naught amidst the rise of jet-powered fighters seen in the period immediately following the end of World War 2.
The J21 was unique for its time aloft in that it began operational service as a prop-driven fighter and ended its days as a jet-powered mount. Few other aircraft were led down this development path and managed to see useful service lives.