Why didn't the Viggen have a Rolls-Royce engine?

Saab originally wanted the British Rolls-Royce Medway as the Viggen's powerplant. That engine had been selected because it was then considered an ideal basis for a fully modulated afterburning supersonic engine. Its development ended up cancelled when, during development, the de Havilland Trident, the aircraft it was meant to debut on, was reduced in size.

Instead of the Medway, Saab adopted a licence-built production version of the American Pratt & Whitney JT8D engine, the Volvo RM8. It was essentially a heavily modified variant of the JT8D that powered 1960s airliners, to which an afterburner was added for the Viggen. The RM8 was thoroughly redesigned, with new materials to allow flight at Mach 2, a Swedish-built afterburner and a fully variable nozzle. It was the world's second operational afterburning turbofan and the first fitted with a thrust reverser. The thrust reverser made it possible to achieve the short landing distances that were sought.

On 8 February 1967 the first of the seven prototypes eventually built made its first flight, in line with the established development schedule. In November 1977 the first series-production JA 37 Viggen made its first flight. By the mid-1980s, the Swedish air force's Draken had been largely replaced by the more advanced JA 37 Viggen fighter.