The effort to develop an all-weather interceptor began in full in 1964 and this produced the first Chinese-designed and built jet fighter to combat new, high altitude threats such as the B-58 Hustler bomber, F-105 Thunderchief fighter-bomber and Lockheed U-2 spy plane. In 1964 the People's Liberation Army Air Force requested an aircraft from Shenyang Aircraft Corporation and the 601 Institute to develop a fighter/interceptor to counter bombers and spy planes as the newly introduced Chengdu J-7 (a reverse-engineered MiG-21) was incapable of doing so.