The Tupolev Tu-4

1st displayed during a flyover at the Aviation Day parade in August 1947 at the Tushino Airport in Moscow. Three aircraft flew overhead. It was assumed that these were merely the three B-29 bombers that were known to have been diverted to the USSR during World War II.

 Minutes later a fourth aircraft appeared. Western analysts realized that the Soviets must have reverse-engineered the B-29 that the United States had refused to lend-lease them in the first place. Twice

http://therealsteelman.tumblr.com/day/2015/05/17

However, on four occasions during 1944, individual B-29s made emergency landings in Soviet territory and one crashed after the crew bailed out.

 In accordance with the Soviet–Japanese Neutrality Pact, the Soviets were neutral in the Pacific War and the bombers were therefore interned and kept by the Soviets. Despite Soviet neutrality, America demanded the return of the bombers, but the Soviets refused.

 Three repairable B-29s were flown to Moscow and delivered to the Tupolev OKB. One B-29 was dismantled, the second was used for flight tests and training, and the third one was left as a standard for cross-reference.

On August 8, 1945 two days after the U.S. atomic bombing of Hiroshima, the fourth B-29 was returned to the US along with its crew.

Stalin told Tupolev to clone the Superfortress in as short a time as possible instead of continuing with his own comparable ANT-64.

The reverse-engineering effort involved 900 factories and research institutes, who finished the design work during the first year; 105,000 drawings were made. By the end of the second year, the Soviet industry was to produce 20 copies of the aircraft ready for State acceptance trials.

 The Soviet Union used the metric system, thus sheet aluminum in thicknesses matching the B-29's imperial measurements were unavailable. The corresponding metric-gauge metal was of different thicknesses. Alloys and other materials new to the Soviet Union had to be brought into production. Extensive re-engineering had to take place to compensate for the differences, and Soviet official strength margins had to be decreased to avoid further redesign, yet despite these challenges the prototype Tu-4 only weighed about 750 lb more than the B-29, a difference of less than 1%.

The Tu-4 first flew on 19 May 1947, piloted by test pilot Nikolai Rybko.[19] Serial production started immediately, and the type entered large-scale service in 1949. Entry into service of the Tu-4 threw the USAF into a panic, since the Tu-4 possessed sufficient range to attack Chicago or Los Angeles on a one-way mission, and this may have informed the maneuvers and air combat practice conducted by US and British air forces in 1948 involving fleets of B-29s.

https://en.wikipedia.org/wiki/Tupolev_Tu-4