The de Havilland Comet first flew in 1949 and entered service with BOAC in 1952, becoming the world's first commercial jet airliner. It was faster, quieter, and pressurized higher than any propeller-driven aircraft. But in 1953 and 1954, three Comets disintegrated in flight. The investigation revealed that the square-cornered windows created stress concentrations that led to catastrophic metal fatigue. The Comet was redesigned with oval windows, but the damage to its reputation was irreversible. Meanwhile, Boeing was developing the 707. The Dash 80 prototype first flew in 1954, and Pan Am introduced the 707 on transatlantic routes in 1958. It had oval windows from the start, swept wings, and four Pratt & Whitney JT3C turbojet engines. The 707 dominated commercial aviation for two decades and became the basis for the KC-135 tanker and the E-3 AWACS. The lesson of the Comet is embedded in every aircraft design curriculum: stress concentrations at sharp corners are unacceptable in pressurized fuselages. Every airliner flying today has rounded windows because of what the Comet taught us.