The second prototype crashed in September 1944 and the program was canceled after the death of Nikolai Nikolaevich Polikarpov, the chief designer of his eponymous OKB, earlier that year.

Mikhail Yangel was appointed head designer, but his job was complicated by multiple changes in the role of the aircraft from escort fighter to interceptor, dive bomber, and eventually reconnaissance.

[1] The prototype, internally designated as aircraft or TIS "A", was a low-wing, all-metal, cantilever monoplane with two Mikulin AM-37 engines and a twin tail.

51 attempted to fix the stability problem in late September by increasing the area of the rear fins, but was unsuccessful.

Flight testing continued in October in Novosibirsk, to where the LII (Russian: Лётно-исследовательский институт—Flight Research Institute) had been evacuated.

The engines were optimized for low altitudes and the aircraft could only reach a maximum speed of 535 km/h (332 mph) and a ceiling of 6,600 metres (21,700 ft).

[5] Data from Gunston, The Osprey Encyclopedia of Russian Aircraft 1875-1995[6]General characteristics Performance Armament