 The J-21 Jastreb was developed as a replacement for the Republic F-84 Thunderjet, which had been the most commonly used turbojet fighter-bomber aircraft of the Yugoslav Air Force until 1967.On the basis of the G-2 Galeb, the J-21 Jastreb was developed as a single seat ground attack variant, flying for the first time on 19 July 1965.Pilots sit on licence-built Folland Type 1-B ejection seats under individual canopies hinged on the starboard side in un-pressurised cockpits.