This is an example of the Royal Air Force officer's service cap used during World War II.
This particular cap was worn by Pilot Officer William O'Regan of 71 (Eagle) Squadron from July 1941 to September 1942. On September 29, 1942, 71 Squadron was transferred into the United States Army Air Forces and became the 334th Fighter Squadron, 4th Fighter Group. During his service with the USAAF, O'Regan was awarded the Distinguished Flying Cross and Air Medal with 2 Oak Leaf Clusters.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.