My dad spent almost all of the war years working on radar, radar countermeasures, and microwave battlefield communications. He was a Physics grad student at Cal Tech when the war broke out. He was recruited by what later became the Manhattan Project and by the Rad Lab at MIT. Chose to go with Rad Lab and for about 4 years was at the American British Laboratories in Malvern, UK, supporting the electronic gear of the 9th AAF.
By the time the Allies landed at Normandy, the Luftwaffe had been pretty much neutralized so he was shifted from radar to microwave communications. After the liberation of Paris, he installed equipment at the top of the Eiffel tower, and as the family story goes, the brass would have to get a pass from him to take their French girlfriends up in the tower. As the Allies moved east, he followed, installing what was basically the precursor to today's cell towers on the high points, and as a consequence he was near the front during the Bulge.
So he didn't actually serve, but he wore a uniform (called a TO, Technical Observer with assimilated rank of Captain) and was intimately involved with those in combat. His mother saved all his letters home and I am working on making them accessible via the Web.
Warren