The Yermolayev Yer-2 () was a long-range Soviet medium bomber used during World War II. It was developed from the Bartini Stal-7 prototype airliner before the war. It was used to bomb Berlin from airbases in Estonia after Operation Barbarossa began in 1941. Production was terminated in August 1941 to allow the factory to concentrate on building higher-priority Ilyushin Il-2 ground-attack aircraft, but was restarted at the end of 1943 with new, fuel-efficient, Charomskiy ACh-30B aircraft Diesel engines. Although designed as a long-range medium bomber it was flown on tactical ground-attack missions during the Battle of Moscow with heavy losses. The survivors were flown, in ever dwindling numbers, until August 1943 when the last examples were transferred to schools. However, the resumption of production in 1943 allowed the aircraft to resume combat operations in April 1945. The Yer-2 remained in service with Long-Range Aviation until it was replaced by four-engined bombers such as the Tupolev Tu-4 at the end of the 1940s. Development Roberto Bartini had designed and built the Stal-7 airliner whilst he was the chief designer at the ZOK NII GVF ( — "Factory for Special Constructions at the Scientific Test Institute for the Civil Air Fleet"). The performance of the Stal-7 was exceptional, particularly in respect to its payload: at gross overload weight over 56% of the total weight was payload. During flight trials with maximum all-up weight the prototype crashed on takeoff in early 1938, resulting in the arrest of Bartini and his imprisonment in a Siberian Gulag in February 1938. The Stal-7 lay unrepaired until Vladimir Yermolaev was appointed as chief designer at OKB-240 after Bartini's arrest, with the task of transforming the Stal-7 design into a long-range bomber, a task made easier since Bartini had reserved space for a bomb bay in the fuselage. After repair the Stal-7 carried on with the flight-test programme, including a record-breaking nonstop flight on 28 August 1939 when it flew Moscow—Sverdlovsk—Sevastopol—Moscow; a distance of at an average speed of . Preliminary design of the DB-240 (—"long-range bomber"), as the bomber version was designated, was completed by the beginning of 1939 and the construction of two prototypes began the following July. The DB-240 retained little apart from the general layout of the Stal-7 as the structure was almost completely redesigned. The pilot's cockpit was offset to port to improve his downward view and the navigator/bomb aimer sat in the