The second aircraft still featured the fixed gear, but had the improved Shvetsov M-63 radial of 1000 horsepower 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.