The Comet was a gorgeous airliner, but its square-window configuration led to disaster as one aircraft after another suddenly began falling out of the skies. After extensive testing (of a pressurized fuselage submerged in a huge water tank), DeHavilland concluded that minor stress cracks that began in the corners of the windows ultimately lead to in-flight disintegration of the entire airframe! By the time they'd fixed the problem, it was too late for the British airliner industry as Boeing's 707 began gobbling up any/all available contracts!
