Powered by a single BWB licence-built Rolls-Royce Viper Mk531 the Jastreb has a conventional aluminium alloy stressed skin structure with few or no special features.

The levered undercarriage legs and relatively low pressure tyres allow the Jastreb to operate from un-prepared strips or rough surfaced airfields.

Compared to the Galeb, the Jastreb has a strengthened structure, allowing more weapons to be carried, including three 12.7 mm (0.5 in) Browning AN/M3 machine guns, mounted in the nose of the aircraft.

[2][3][4][5][6][7][8] Six J-21 Jastrebs of the Republika Srpska Air Force were engaged by USAF F-16s during Operation Deny Flight for violating the NATO-enforced no-fly-zone, in what is known as the Banja Luka incident.

On 15 November, during the Battle of the Dalmatian Channels, at 9:28 a.m. three Yugoslav Air Force J-21 Jastrebs flew low over Brač and Šolta; minutes later, two were reportedly shot down by anti-aircraft artillery.