The Su-39 aircraft was a further development of a specialized "anti-tank" version of the Su-25T attack aircraft. The task of fighting tanks for the Su-39 was assumed to be important, but not dominant, like the Su-25T. It was assumed that the aircraft would be able to effectively destroy ships in coastal areas, transport and front-line aircraft of the enemy. The weaponry and avionics of the aircraft underwent significant processing.
The modernized aircraft was equipped with a Spear-25 radar station in a hanging container, which radically changed its capabilities. In particular, the Su-39 was supposed to conduct a full-fledged air battle, for which the R-73, R-27 and R-77 missiles were included in its arsenal of air-to-air missiles [with maximum launch ranges of 20/40, 65/80 and 80/110 km respectively. Anti-ship Kh-31A with a launch range of up to 110 km was designed to destroy enemy ship groupings. The fight against the enemy radar was carried out by anti-radar Kh-25MPU and Kh-31P. The arsenal of assault weapons was replenished with the high-precision missile system "Whirlwind".
By design, the Su-39 aircraft is basically identical to the Su-25UB aircraft, designed for combat training flights. But since the Su-39 is a single-seat aircraft, the place of the second crew member was occupied by a soft fuel tank, and additional avionics equipment was placed in the niche located above it. The dimensions of the attack aircraft are quite compact, which makes it maneuverable, difficult to hit, capable of flying at low altitudes, avoiding obstacles and terrain features.
