Don't think they had one .This position wasn't created until 1947 when the Navy, Army, and newly created Air Force were merged into the new National Military Establishment. Before that they had a secretary of war .George C Marshall rings a bell
Wikipedia will tell you more. http://en.wikipedia.org/wiki/United_States_Sec retary_of_Defense
George Catlett Marshall was certainly the Chief of Staff of the United States Army and later became Secretary of Defense (1950-51) but the Secretary of War during WWII was Henry Lewis Stimson.
I doubt that anyone has a secretary of war these days. It's all secretaries of defence, because as we know countries never go to war. So secretary of defence was a politically correct term decades before political correctness.