In the aftermath of the Second World War, Sweden required a strong air defence utilising the newly developed jet propulsion technology. According to aviation author Bo Widfeldt, there was a perception that Sweden had not kept up with wartime innovations and technical progress, and that Saab was eager to make aeronautic advances, particularly in terms of developing jet propulsion. Accordingly, project "JxR" was initiated in the final months of 1945, leading to the requirements being drawn up in October 1945. This led to a pair of proposals being issued by the Saab design team, led by Lars Brising. The first of these, codenamed R101, was a cigar-shaped aircraft which bore a resemblance to the American Lockheed P-80 Shooting Star. The second design, which would later be picked as the winner, was the barrel-shaped design, codenamed R 1001, which proved to be both faster and more agile upon closer study.
The original R 1001 concept had been designed around a mostly straight wing, but after Swedish engineers had obtained German research data on swept-wing designs, the prototype was altered to incorporate a 25 degree sweep. A member of the Saab engineering team had been allowed to review German aeronautical documents, stored in Switzerland following their capture by the Americans in 1945; these files had clearly indicated delta and swept-wing designs to have the effect of "reducing drag dramatically as the aircraft approached the sound barrier." In order to make the wing as thin as possible, Saab elected to locate the retractable undercarriage upon the aircraft's fuselage rather than upon the wings.
Extensive wind tunnel testing performed at the Swedish Royal University of Technology and by the National Aeronautical Research Institute had also influenced aspects of the aircraft's aerodynamics, such as stability and trim across the aircraft's speed range. These tests had determined the required slenderness of the fuselage in order to ensure compatibility with the targeted critical Mach number, as well as supporting the use of a straight-through airflow system to ensure the maximum attainable thrust, in addition to the advantages of its ease of development. For the reason of lateral stability during take-off and landing, automatically-locking leading edge slots, which were interconnected with the flaps, were also deemed necessary. In order to further test the design of the swept wing, it was decided to modify a single Saab Safir, which received the designation Saab 201, with a full-scale wing for a series of flight tests. The first 'final' sketches of the aircraft, incorporating the new information, was drawn in January 1946.
The originally envisioned powerplant for the type was the de Havilland Goblin turbojet engine. However, in December 1945, information on the newer and more powerful de Havilland Ghost engine became available. This was deemed to be ideal for Saab's in-development aircraft as not only did the Ghost engine have provisions for the use of a central circular air intake, the overall diameter of the engine was favourable for the planned fuselage dimensions; thus, following negotiations between de Havilland and Saab, the Ghost engine was selected to power the type instead.
Despite early doubts over the availability of a suitable aluminium alloy, similar to the American 75S alloy, Svenska Metallverken was able to manufacture the sufficient grade of sheet metal, equivalent in strength to its US-based counterpart, albeit requiring the use of significantly larger sections than had typically been employed in aircraft construction. The structure employed a complicated mixture of stressed skin and heavy frames in order to meet conflicting requirements on space, strength, rigidity and accessibility.