(Tu-4, an exact copy of the B29)
Damaged American B-29 bombers, trying to find a safe place to land after a bombing raid on Japan, occasionally flew north to the Russian port of Vladivostok. But because the Russians hadn't declared war on Japan and were maintaining neutrality, the planes were seized and the crews "interned" in camps. They were eventually allowed to "escape" and find their way home through Iran, but the Russians kept the planes.
Of course we sent them lots of Lend-Lease, but not the B-29.