Complex sentence: When the operation was still in the development stage the leading candidates to command the group designated to drop the atomic bomb had been Armstrong and Colonel Roscoe C. Wilson, the Army Air Force project officer providing liaison support to the Manhattan Project.
Split into simpler sentences: When the operation was still in the development stage, Armstrong and Colonel Roscoe C. Wilson were the leading candidates to command the group who was designated to drop the atomic bomb. Wilson was the Army Air Force project officer who provided liaison support to the Manhattan Project.