The Republic F-105 Thunderchief, commonly known as the "Thud" by its crews, was a single-seat supersonic fighter-bomber used by the United States Air Force. The largest single-engined fighter ever employed by the service, it saw extensive use during the Vietnam War in both strike and SEAD roles. Despite the fighter designation, the primary intended role of the F-105 was high-speed low-altitude penetration into enemy airspace with a single nuclear bomb in the internal bomb bay. First flying in 1955, the Thunderchief entered service in 1958. Typical of advanced aircraft at the time, early models suffered from multiple problems with the electronics, the fuel system, and the engine. Nevertheless, the single-seat single-engine F-105 could carry a greater bomb load than the four-engined ten-man heavy bombers of World War II to a greater distance and at much higher speed. As the result, the F-105 became the primary ground attack aircraft in the early stages of the Vietnam War, flying over 20,000 missions at the cost of 334 combat losses. Two-seat F-105F and F-105G Wild Weasel variants were the first dedicated anti-air defense platforms fighting against the Soviet-built S-75 Dvina (SA-2 Guideline) surface-to-air missiles. Two Wild Weasel pilots earned the Medal of Honor. Although it weighed 50,000 pounds (22,680 kg), the F-105 could exceed the speed of sound at sea level and Mach 2 at high altitude, and could carry up to 14,000 pounds (6,700 kg) of bombs and missiles in addition to an internal 20 millimeter M61 Vulcan cannon. The Thunderchief was retired in 1984, with a total of 833 aircraft built.