Near the end of WWII, in late 1944, three US B-29 Superfortresses were forced to land in Russia near Vladivostok. The Russians seized them but returned the crews. They had been trying to get B-29's through the Lend-Lease program but weren't allowed to get any.
So the Tupolev design bureau took them, disassembled them and began making nearly exact copies, called the TU-4.
Years later an engineer that had worked on the original B-29 design was able to inspect a TU-4, and found that the copies were so exact the Russians had even copied a couple of holes in the main spar carry-through that were mistakes in the original design but had no use.