The two-seat, all-weather Su-24 tactical bomber, which can operate around the clock, was developed in order to counter the USAF F-111 multi-role tactical fighter. Development of the Su-24 started in 1961 under the internal codename T-6. The first prototype, the T-6-1, was completed in May 1967 and was first operated by Soviet test pilot Vladimir Ilyushin on July 2. It was the first Soviet tactical attack aircraft to feature an integrated digital navigation/attack system. On February 4, 1975, the Su-24 was accepted into service, kicking off an important stage in the development of the USSR's national aircraft industry.