A few B-29s landed in Siberia after sustaining damage over Japan. At the time the Soviets were neutral with respect to Japan and so the planes and crew were interned.

Its best to think of World War Two as two separate, but parallel conflicts. The Soviets were at war with Germany, but they were neutral with respect to Japan.

And as a neutral power, the Soviet Union had the right to impound any war weapons which sought refuge within the Union. So, when American bombers would get damaged during a raid over Japan, the pilots would sometimes land in Soviet territory (it was that or crash out in the Pacific somewhere). But now these B-29s, advanced pieces of hardware, were in the Soviet Union. The pilots were usually returned after a few days or weeks of detention, but the Soviets kept a few of the B-29s. I think it may have only been 2-3 planes, but it was enough. Soviet engineers went over the plane, inch by inch, and recreated the bombers down to every bolt and seam. The Tupolev Tu-4 is the fruit of that work. Compare it to an American B-29

Allied flyers who were forced to land in Soviet controlled territory were sometimes treated with great suspicion, if not downright hostility. Some of them were actually treated as POWs. During the last months of the war some US B29 crews chose to land in Russia after missions over northern Japan.