The Borovkov-Florov D was an experimental fighter aircraft designed in the USSR from 1940.
The 'D' was designed as a mixed power fighter with a piston engine and Merkulin ramjet booster operating in the same duct.
Similar aircraft were built later in the German-Soviet War using the main engine to drive a propeller and a compressor to supply air to a ramjet/afterburner booster, both the Su-5 and MiG-13 were produced in limited numbers but the performance gains were limited and soon eclipsed by turbo-jet engines.
The 'D' was to have been a gull-winged monoplane with high set wing, of stressed skin construction with exceptionally smooth skin stabilised by underlying corrugated structure.
A heavy armament of two 37 mm (1.457 in) Nudelman-Suranov NS-37 cannon and two 20 mm (0.787 in) ShVAK cannon was included, but all work was abandoned with the German invasion of 1941.