Lieutenant-General Lesley J. McNair
Highest ranking American killed was Lt. General Lesley McNair,
killed by the US Army Air Corps.

General McNair was the highest ranking American General to be killed in action in WW2.  He was killed by American bombs shortly after D-Day, when bad weather caused the bombers to drop their loads "blind".

Lt. Gen. Lesley J. McNair, commanding general of the Army Ground forces and one of the Army's highest ranking officers, paid a brief visit to Camp Atterbury Friday afternoon, landing by plane at Atterbury Army Air field, here.

The Camp Atterbury public relations office stated that General McNair, who has headquarters in Washington, D. C., was on a' routine tour of inspection.

General McNair was greeted at the local airfield by Brig. Gen. Herbert P. Perrin, assistant commander of the 106th Division at Atterbury; Col. Richardson L. Greens, commanding officer of the Eighth Detachment of Special Troops, Second army, stationed at Atterbury; Col. W Eaton M. Modisette, Atterbury commander, and Capt. George B. Ober, commanding officer of the Air field.

General McNair was at Atterbury about one and one-half hours. 

He was accompanied by a number of staff officers. The Army Ground forces commander was commander of R. O. T. C. at Purdue university from 1924 to 1928.

Lt. Gen. Leslie McNair was one of the highest ranking American officers killed in World War II. McNair had been commander of Army ground forces and was responsible for training of all components of the active Army, Army Reserve, and National Guard. He wanted a field command but never received one. As frequently as he could, he visited the fronts and was wounded in Tunisia. He was made commander of the mythical 1st Army Group, replacing General Patton McNair was observing the 30th Infantry Division’s preparations for deployment to St. Lo in 1944 when the Army Air Corps accidentally dropped bombs on his position and he was killed. He was posthumously promoted to full general in 1945.

Ironically, his son, Colonel Douglas McNair, chief of staff of the 77th Division, was killed two weeks later by a sniper on Guam.

Tom Cagley

http://www.wwiihistorymagazine.com/2005/july/col-insight.html