extensively glazed nose with a ShKAS machine gun, the radio operator sat below and to starboard of the pilot and the dorsal gunner in a partially retractable turret with one Berezin UBT machine gun. Another ShKAS was fitted in a ventral hatch. Up to of bombs could be carried in the bomb bay and two bombs could be carried externally. Up to of fuel could be carried. The DB-240 had been designed to use the experimental Klimov M-106 V12 engines, but the less-powerful Klimov M-105 engine had to be substituted because the M-106 was not available. The DB-240 prototype flew for the first time on 14 May 1940 and began its State acceptance tests on 27 September 1940. The weaker engines prevented the DB-240 from reaching its designed performance. It could only attain at instead of the expected at . Its defensive armament was deemed inadequate and other problems included an excessively long take-off run and engine defects. However, these did not offset its virtues of a heavy bomb load and long-range ( carrying of bombs). It was ordered into production at Factory No. 18, in Voronezh, as the Yermolayev Yer-2. Manufacture began in March 1941, with approximately 50 aircraft delivered by 22 June 1941. These aircraft were about slower than the prototype and their normal weight increased to . Production was terminated in August 1941 with only 128 built to allow the factory to concentrate on the higher-priority Ilyushin Il-2 ground-attack aircraft. A Yer-2 was modified with experimental Mikulin AM-37 engines, a reinforced undercarriage, armored seats for the navigator and gunner, and 12.7 mm UBT machine guns in place of its original ShKAS weapons. It first flew in July 1941 and was able to reach at , but the range was reduced to ( carrying of bombs. One significant problem with this version was the excessive takeoff roll which hindered operations from grass airstrips. The engine was unreliable, however, and had cooling problems that the Mikulin OKB did not have the resources to resolve so it was cancelled in October when the factory was forced to evacuate from Moscow by the German advance. The Charomskiy M-40F Diesel engine was also evaluated in a Yer-2 in 1941. This engine, like all Diesels, offered a greatly reduced fuel consumption compared to a standard gasoline-powered engine, but at a great penalty in weight. These engines increased the gross takeoff weight to which