The B-58 was designed to fly at high altitudes and supersonic speeds to avoid Soviet interceptors. But with the Soviet introduction of high-altitude surface-to-air missiles, the B-58 was forced to adopt a low-level-penetration role that severely limited its range and strategic value. It was never used to deliver conventional bombs. The B-58 was substantially more expensive to operate than other bombers, such as the Boeing B-52 Stratofortress, and required more frequent aerial refueling. The B-58 also suffered from a high rate of accidental losses. These factors resulted in a relatively brief operational career of ten years. The B-58 was succeeded in its role by the smaller, swing-wing FB-111A.