US pilots flew Soviet planes in Spain against Francos fascists, some of them even became ace status.
The most famous one was Frank Glasgow Tinker, who was the first pilot ever to shoot down an bf-109, unfortunately he did not live long enough to fight in WW2, as he apparently committed suicide.
http://www.youtube.com/watch?v=3mT153c1Ev8
http://www.encyclopediaofarkansas.net/encyclopedia/entry-detail.aspx?entryID=4581