Lets dispell some rumors.
1) Flying wings were in development with various companies both in the US and in Germany before the war started. Although the Germans made significant strides, the concept that one country borrowed the idea from the other doesn't entirely ring true. Collaboration of general principles long before the war started may be true...but in this instance US technology didn't gain anything from German flying wings.
The V-2 rockets and whatnot were intently studied by both Russians and Americans and that did eventually lead to the Apollo missions.
2) It'd be very hard to say that the F-15 and the Su-27 and its varients were copies of each other (the 27 being a copy of the 15 is the implication). If you look at the development history of the aircraft its much easier to notice that the Su-27's evolution from drawingboard to finished product borrowed only the concept of heavy and light fighter from the Americans (the MiG-29 being the light and the Su-27 being the heavy countering the light F-16 and the heavy F-15). Undoubtedly, F-15 designs were influenced just as much by the MiG-25 as the Su-27 was influenced by the F-15. In terms of performance, the Su-27 is much (much!) more manuverable while the F-15 is faster. Hardly a copy.
3) The B-29 was indeed copied under orders from Stalin. I think it was called the Tu-4. It was copied, rivet for rivet, on the B-29's design using parts that had been captured and aquired during the war (ie. crashed wrecks).
So there
Point overall being that most designs, although influenced by competing designs, are not generally exact copies (except in the Tu-4/B-29 case). Most of the time, the concepts are either the correct ones and/or are moderatly influenced because of the competing design.
The F-86 Sabre is not a copy of the MiG-15 but when swept wings proved useful on the MiG, efforts were made to ensure that the Sabre would have that too.
