Sweden aircraft design has never been lacking in innovation and probably the most extreme example of this is the Saab 21, the world's only major combat aircraft to be built in both piston and jet engined versions. It was designed during World War II as a single engined fighter with a twin boom and pod configuration (a rarity for an aircraft with only one engine) with propulsion provided by a Daimler-Benz engine driving a pusher propeller, something that required an ejector seat: it was the second fighter ever (after the He 219) with one. It was later decided to be equipped with a jet engine with the new aircraft retaining 50 percent commonality with its piston predecessor and with the only major difference being being a tailplane mounted at the top of the twin fins and a lowered undercarriage. Despite (or perhaps due to) such a radical design, the Saab 21 had generally unsatisfactory performance as a result of low speeds and poor maneuverability and was phased out of Swedish service by 1954.

First flight of the prototype took place on 30 July 1943 with deliveries beginning in mid-1945 of the piston engined version designated J 21A-1. Variants included the slightly improved J 21A-2 and the A 21-3 attack platform. Jet engined versions entered service as the J 21R which came in two versions: the J 21RA with British built Goblin II engines, and the J 21RB with locally built Goblin IIIs. These were ultimately converted into A 21R attack aircraft after their failures as fighters.