 Three additional fuel tanks, with a capacity of 700 liters (154 imp gal; 185 U.S. gal) were fitted in the prototype, one in the fuselage bomb bay and the other two replaced the ventral gunner's position. The nose armament was reinforced with an additional 12.7-millimeter (0.50 in) Berezin UBK machine gun with 150 rounds and a fixed 7.62-millimeter (0.300 in) ShKAS machine gun with 250 rounds was added in the tail cone. Removal of two of the fuselage bomb racks reduced the maximum bomb load to 700 kilograms (1,543 lb), one 250-kilogram (551 lb) bomb on each of the fuselage racks and one 100-kilogram (220 lb) in each of the engine nacelles. The electric bomb release system was removed and the bombs had to be dropped using the mechanical system, initially designed as the emergency system. The dive brakes under the wings were also removed. The BSBbis radio was exchanged for the RSI-4 model commonly used in single-seat fighters and the radio direction finder was also removed to save weight. The prototype weighed 5,890 kg (12,985 lb) empty and it had a normal take-off weight of 7,800 kg (17,196 lb), slightly heavier than the version of the Pe-2 then in production. During testing it demonstrated a maximum speed of 530 km / h (329 mph) at 5,000 meters (16,404 ft), a service ceiling of 9,000 meters (29,528 ft) and a maximum range of 2,150 km (1,336 mi). This was considered adequate and Factory (Russian: Zavod) Nr. 39 in Moscow was ordered on 14 August to build five pre-production aircraft for delivery by 25 August. This process proved difficult because drawings had not been made for many of the new parts and they had to be fitted by hand, slowing the production rate.
