The Ilyushin I-21, (Istrebitel-21, fighter-21), was a single-engined, single-seat fighter produced in the USSR in 1936-7 in response to a government specification.
It was an all-metal, low-wing, cantilever monoplane with retractable undercarriage and closed cockpit. The wings were sharply tapered with straight trailing edges, which introduced sweep back on the leading-edge. Two specially modified Mikulin AM-34RNF, (also written AM-34FRN), liquid-cooled V-12 engines provided the power via a non reducing gearbox, which was used only to raise the thrust line, and enable the use of shorter undercarriage legs.
Cooling for the first prototypes engine used an evaporative system which dispensed with radiators and used condensers built into the surface of the wing centre section. The second prototype was fitted with a conventional cooling system using ethylene glycol coolant and a retractable radiator. Flight trials started but soon showed that the evaporative cooling system was inadequate with both the engine and the wing centre section overheating. Ilyushin was also very aware of the vulnerability of the system to combat damage, but he was ordered by GUAP to use this system. Further development was halted in 1939 and the I-21 designation was re-used for the Pashinin I-21.