Borovkov and Florov drew heavily on their experience working in the Polikarpov I-16 program, producing 'Prototype No.7211' (Izdeliye 7, Zavod 21, Samolyet 1 – article 7, factory 21, aircraft 1).

The goal was the smallest possible aircraft with the most powerful engine then available in the Soviet Union, the 850 hp (630 kW) Mikulin M-85 14-cylinder radial, a licensed-built Gnome-Rhône K14 Mistral Major.

Retaining the open cockpit, it featured a smooth close fitting cowl, fixed landing gear, and was fully armed with four ShKAS 7.62-mm machine guns in the forward fuselage.

The second aircraft still featured the fixed gear, but had the improved Shvetsov M-63 radial of 1,000 hp (750 kW) and the lower wings were strengthened to accept either two FAB-250 bombs or Merkulov DM-4 ramjets for a rapid climb on takeoff, with a first flight in late April or early May 1939 proving its high performance - maximum airspeed 416 km/h (258.5 mph) at 5000 m (16,404 ft) and 18 m/s (3,543 ft/min) rate of climb after take-off.

The fourth production prototype featured a fully enclosed cockpit, a geared M-63 engine and a large smooth spinner that gave the little airplane a dart-like look.