A prototype test aircraft, the Messerschmitt Me P.1101, was built to research the tradeoffs of the design and develop general rules about what angle of sweep to use.[45] When it was 80% complete, the P.1101 was captured by US forces and returned to the United States, where two additional copies with US-built engines carried on the research as the Bell X-5.[46] Germany's wartime experience with the swept wings and its high value for supersonic flight stood in strong contrast to the prevailing views of Allied experts of the era, who commonly espoused their belief in the impossibility of manned vehicles travelling at such speeds.[47]