This is an example of the Royal Canadian Air Force officer's service cap used during World War II.
This particular cap was worn by the donor Flight Lieutenant William Jones of 121 (Eagle) Squadron from June 1941 to September 1942. Jones flew Spitfires based at Kirton-in-Lindsey. On September 29, 1942, 121 Squadron was transferred into the United States Army Air Forces and became the 335th 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.