 A total of 17 YF-104As were ordered by the USAF on 30 March 1955 for further flight testing.The first of them flew on 17 February 1956 and, with the other 16 trial aircraft, was soon carrying out aircraft and equipment evaluation and tests.Lockheed made several improvements to the YF-104A throughout this testing period, including strengthening the airframe, adding a ventral fin to improve directional stability at supersonic speed, and installing a boundary layer control system (BLCS) to reduce landing speed.