Just after WWII The USSR built carbon copies of the B29 from a plane that had been left/abandoned in the Soviet Union....right down to a repair patch on the tail section. By then the U.S. had developed and deployed the B52.
Our then-frenemies, Russia, was the designated alternate landing site for any B29 that potentially could not make it back to base after a nice BBQ run over Japan. 8 B29s landed in Russia, and we never got them back.
They also did a lot of reverse engineering using a captured 86 Sabre during the Korean War. The US got its revenge later on, when it captured a MiG brought be a defector, declared it a worthless pile of dog feces, and returned the plane to Russia in a lot of boxes filled with airplane parts.
At least weren't as dumb as the UK, who gift-wrapped their jet engine technology and shipped it to Russia.