After the end of the Second World war, SAAB 21 was modified to accomodate a jet engine instead of the piston one. First jet-powered SAAB 21R was flown on March 10, 1947. A 21R were built. S 91 is a single-engine monoplane. Low-set tapered wings are unswept.
Tailplane is rectangular.
Landing gear with nose wheel is not retractable. First S 91 prototype was flown on November 20, 1945. S 91 was used by various organizations in Austria, Ethiopia, Finnland, Norway, Sweden, and Tunisia. The fuselage is oval with a wide, rounded tail fin with a small dorsal extension. Several square windows are located on either side of the fuselage.