上世纪50年代,在朝鲜战争以及渐趋恶化的冷战形势的推动下,新式后掠翼喷气战斗机呈现出突飞猛进的发展势头,提升高空、高速作战能力成为当时战斗机和轰炸机发展的着眼点。空军庞大的轰炸机部队使美国在战略力量对比上处于优势,但在60年代的越南战争中,面对苏制地空导弹,高空飞行轰炸机的生存能力面临前所未有的挑战。“越高越快越好”这个旧信条很快被“越低越快 In the 1950s, under the impetus of the North Korean war and the deteriorating cold war situation, the new swept back wing jet fighter showed a rapid development momentum. The enhancement of altitude and high speed combat capability became the focus of the development of fighter jets and bombers at the time. The Air Force’s huge bomber force gave the United States a comparative advantage in strategic forces. However, in the Vietnam War in the 1960s, the survivability of its high-altitude flight bombers faced unprecedented challenges in the face of Soviet-made ground-to-air missiles. The old creed of ”higher, faster, faster, better,“ is sooner ”lower, faster."