The F- series number sequence used in USAF was a continuance of the pre-USAF pursuit aircraft (P- series) numbering, stretching back as far as to the 1920s. The numbering would continue sequentially up to the General Dynamics F-111, and after this number the 1962 United States Tri-Service aircraft designation system restarted the numbering back from 1. The McDonnell Douglas F-4 Phantom II fighter-bomber was briefly known as the F-110 Spectre. The USAF continued the naming convention with the CONSTANT PEG program as an operations security measure. The last known aircraft with a "Century Series" designation is the Lockheed F-117 Nighthawk stealth attack aircraft.