Original sentence: The second aircraft still featured the fixed gear, but had the improved Shvetsov M-63 radial of 1,000 hp and the lower wings were strengthened to accept either two FAB-250 bombs or Merkulin DM-4 ramjets for a rapid climb on takeoff,witha first flight in late April or early May 1939 proving its high performance - maximum airspeed 416 km/hr (258.5 mph) at 5000 m (16,404 ft) and 18 m/s (3,543 ft/min) rate of climb after take-off.This improved the top speed over the fixed gear version by 20 mph (32.5 km/h)
Edited sentence: The second aircraft still featured the fixed gear, but had the improved Shvetsov M-63 radial of 1,000 hp and the lower wings were strengthened to accept either two FAB-250 bombs or Merkulin DM-4 ramjets for a rapid climb on takeoff,witha first flight in late April or early May 1939 proving its high performance - maximum airspeed 416 km/hr (258.5 mph) at 5000 m (16,404 ft) and 18 m/s (3,543 ft/min) rate of climb after take-off.This improved the top speed over the fixed gear version by 20 mph (32.5 km/h)