If I remember the story correctly, three B-29's had to make a forced landing in Russia and because Russia was not at war with Japan, they were confiscated for the duration of the war.
They were reverse engineered to such detail that even some flaws were recreated. The engineers were scared to death that Stalin would punish them if the copies were not 'exactly' as the original - flaws and all. It was an amazing feat of engineering and fabrication to copy them in such a short time.
The B-29 was so advanced for its time that some of the systems had to have seemed like they were from another planet. The automated gun turrets and engine management systems astounded the engineeers. And Russian engineers have never been slouches.
