Soviet aeronautics engineer Vladamir Yermolaev (1909-1944) served as the lead on what became the "Stal-7" aircraft under the Bartini (Robert Ludvigovich Bartini, 1897-1974) brand label in the pre-World War 2 (1939-1945) period. The twin-engine Stal-7 was developed into a single working prototype example intended to serve as a passenger airliner / transport to satisfy a standing commercial Aeroflot requirement. Due to its impressive performance in testing, this same design was selected for conversion into a twin-engine, long-range medium-class bomber under the "Yer-2" designation. Yermolaev went on to become chief operator of the "OKB-240" design bureau in 1939, taking over Bartini's position after he was sent to Siberia.
The Yer-2 was of relatively unique design form for its time with sleek, clean lines throughout. The fuselage was well-rounded and the nose section glazed with the cockpit position stepped. The fuselage tapered towards the tail unit in the usual way and the structure was capped by a split vertical tail fin component featuring separated horizontal planes. The wing mainplanes, of inverse-gull form (ala the German "Stuka" dive bomber), were low-mounted at the fuselage sides and curved at their tips. Each mounted a streamlined nacelle housing Charomskiy ACh-30B V-12 liquid-cooled diesel-fueled piston-driven engines of 1,500 horsepower each. The engines drove three-bladed propeller units of constant-speed design and contributed to the aircraft's top listed speed of 260 miles-per-hour. Range was out to 3,400 miles and the aircraft's service ceiling could reach just under 24,000 feet.
For point-defense, the bomber carried a single 12.7mm UBT series machine gun on a flexible mounting at the glazed nose position, a single 20mm ShVAK automatic cannon in a TUm-5 series dorsal turret emplacement at midships, and a single 12.7mm UBT heavy machine gun at a ventrally-positioned, rear-facing station. To satisfy its bombing requirement, the internal configuration allowed for up to 11,025lb of conventional drop ordnance / bombs to be carried.
The operating crew reached four personnel and the protruding cockpit flight deck was noticeable offset to portside while also being heavily glazed.
A first-flight in its "DB-240" prototype form (two were constructed, powered by M-105 engines of 1,050 horsepower) was recorded on May 14th, 1940 and service introduction followed in 1941 with serial production reaching between 360 and 370 total units. However, production was quickly halted in 1941 - following the German invasion of the Soviet Union through "Operation Barbarossa" in June - as other Soviet Air Force aircraft were to take priority. For its time in the Grand War, the series was put to good use by running sorties directly against the heart of the German empire - Berlin - while launching from bases in Soviet-controlled Estonia. Beyond this, the twin-engine platform was forced into low-level attack work as a tactical bomber for which it was never intended, resulting in unacceptable losses of men and machine alike. However, by this time, the Battle of Moscow was in full swing and any and all aircraft were pressed into whatever role they were needed for.
Yer-2 series bombers served in a frontline capacity into 1943 at which point attrition and obsolescence relegated the series as trainers for future Soviet airmen for the time being. That same year, with gains being made by the Soviets on the ground, serial production was allowed to restart and the bomber flew in its original role into the war's final weeks (April 1945). In all, the series was operated with Long Range Aviation of the Soviet Air Forces of World War 2 through squadrons 327, 329, 420, 421, 747, and 748. Final forms were not retired until well after the war, this being in or around 1950.
The DB-240 prototype pair laid the ground work for the production quality "Yer-2" to be realized, these powered by M-105 engines and production reached 128 total units. Then followed the one-off "Yer-2/AM-37" prototype fitted with 2 x Mikulin AM-37 engines of 1,380 horsepower while "Yer-2/M-40F" served as the basis for the diesel-engined variant (fitting 2 x Charomskiy M-40F units). This led to the "Yer-2/ACh-30B" serial production model which, while showcasing considerable performance increases over the original arrangement, suffered reliability problems.
At least two Yer-2/ACh-30B production aircraft were used as the basis for the "Yer-2ON", a VIP / passenger fast-transport. The "Yer-2N" designated a test article used to study the captured German Argus As 014 pulse jet engine at the end of the war and, similarly, "Yer-2/MB-100" was used to test the Dobrotvorskii MB-100 engine in 1945.
"Yer-4" designated a proposed, improved Yer-2 bomber form. Yer-2 production bombers were to be reworked with ACh-30BF engines providing increased power, given lengthened wings for improved handling, and fielding a better defensive scheme. However, only a prototype of this mark was ever realized and this was tested during 1943.
By this time, the war had moved on without the Yer-2 series as a focal point to future Soviet Air Force plans and its wartime story was more or less written in full.