William Daley attempted to enlisted from Amarillo, Texas, in the Army Aviation Cadet programme but was rejected. He then went to the Clayton-Knight Committee, and having nearly 200 hours of flying time, was accepted for the RAF, where he graduated a a Pilot Officer on 27 June 1941.

On 9 July he arrived in Glasgow, Scotland, and was off to an Operational Training Unit. Upon completion of his training on 1 September he was assigned to No. 121 (Eagle) Squadron flying Hurricanes, soon transferring to Spitfires.He seldom encountered the Luftwaffe in their normal duties of convoy patrol and fighter sweeps. On 24 April, however, he shared in the destruction of a Junkers 52.

He soon downed an Fw-190, and on 27 May he sank a minesweeper and destroyed another Fw-190. In June he became acting Flight Lieutenant. On 29 September 1942, upon transfer to the USAAF, Major Daley became Commanding Officer of 335th Fighter Squadron. In late 1942 he returned to the States and subsequently was assigned as Deputy Commander of the Ninth Air Force, 371st Fighter-Bomber Group.

In September 1944, While taxiing [42-28426], he was rammed by an out of control P-47D [42-26294] and was gravely injured, dying the following day. He had eight air victories with the RAF and the 9th Air Force.

He was awarded the Distinguished Flying Cross, the Air Medal with eight Oak Leaf Clusters, the Purple Heart with an Oak Leaf Cluster and the British Distinguished Flying Cross.