The modern USAF was established on September 18th, 1947 following the conclusion of World War 2 but maintains a history as far back as 1907. Prior to 1947, the USAF operated as the 'United States Army Air Forces' (USAAF) and the service contributed to both World War 1 and the Second World War. The modern Air Force has seen major combat actions since the Korean War and fields a variety of aircraft to cover all manner of mission types.
Formally, USAF rank abbreviations are not displayed with an ending period. Additionally, the USAF does not maintain a 'First Sergeant' rank common to other branches of American military service. Instead, the rank is denoted by the special addition of a diamond within the insignia and observed as a duty / job title. Similarly, the diamond is replaced by a star for the Chief Master Sergeant and Command Chief Master Sergeant ranks.
Note: The Five Star rank is generally reserved for wartime but can also be used as an honorary rank.