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 a closed cockpit. The wings were sharply tapered with straight trailing edges, which introduced sweepback 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.