Answer: Square Windows

Early development in any industry is fraught with hiccups, setbacks, and outright failures, and the development of early jetliners is no exception. The first production commercial jetliner, based on earlier military designs, was the British de Havilland DH 106 Comet. The aircraft paved the way for future jetliners and, through a series of catastrophes, helped shape the design of modern jetliners.

Multiple early DH 106 flights resulted in catastrophic failures of the aircraft. Some were failures occurring during take-off while the others were a result of the fuselage breaking apart mid-flight at high elevations. Although the incidents were initially blamed on pilot error and adverse weather conditions, further research indicated that the accidents occurred as a result of metal fatigue in the frames induced by air pressure (an aspect of metal engineering that was poorly understood at the time given the infancy of the entire aeronautical engineering field).

Among all the points of stress on the frame, it was discovered that the square windows were creating exceptionally high levels of stress at their corners and that the window design was a major contributing factor to the fatigue behind the tragic in-flight accidents that grounded the DH 106 line.

The jet was extensively redesigned with oval windows, and rival manufacturers followed along with oval window designs of their own. To this day, over half a century later, jetliners still have oval windows as a simple precaution against fuselage stress.