Tu-4 the first soviet strategic bomber image - Aircraft Lovers Group

Taken from Wikipedia:

The U.S. refused to supply the Soviet Union with B-29 heavy bombers under Lend Lease, despite repeated Soviet requests. However, on three occasions during 1944, individual B-29s made emergency landings in Soviet territory after bombing raids on Japanese Manchuria and Japan. 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 American demands for their return. Stalin tasked Tupolev with cloning the Superfortress and Soviet industry was to produce 20 copies of the aircraft in just two years. The three B-29s were flown to Moscow and delivered into Tupolev OKB. One B-29 was fully dismantled, down to the smallest bolt, second was used for flight tests and training, and the third one was left as a standard for cross-reference.

I'll just add a little more about this episode. Truly, american bombers performed emercgency landings at the territory of the Soviet Union for bombing of Manchukuo and Japan. Though, from the words of air marshall Golovanow, who was close to Stalin, an event, after which USSR decided to copy B-29 happened in 1945, when one of US bombers lost the course and had to land on the Soviet airfield. Golovanov inspected the plane and rang to Stalin with recommendation to produce THIS plane instead of new Pe-8. Stalin was fast in such questions. This task was given to Tupolev. At first he refused to produce it, but after Stalin promised him to call it in his (Tupolev's) name - he agreed.