This is an example of the Royal Air Force officer's service cap used duirning World War II. This particular cap was worn by Pilot Officer William Wallace of 133 (Eagle) Squadron from June 1941 to September 1942. On September 29, 1942, 133 Squadron was transferred into the United States Army Air Forces and became the 336th Fighter Squadron, 4th Fighter Group.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.