Actually, that honor goes to either Italy or Serbia, depending on what your definition of combat usage is.
I don't doubt you, but we used them against Poncho Villa in 1916 before we entered WW1.
http://www.fac-assoc.org/Ancestry/ancestryofforwardaircontrollersthroughtwwii.htm
http://www.hsgng.org/pages/pancho.htm
I'm wrong in what I said. Others did use the fixed wing before us. Thanks, I was under the wrong impression there.
Red6