In aviation circles, the de Havilland Comet is well known not only for being the first jet-powered commercial airliner, but also for the more ignoble distinction of having crashed numerous times due to structural fatigue caused by pressurization stress concentrated at the corners of its square windows. The general rhetoric about the disasters is that they were an inevitable consequence of the era's poor understanding of metal fatigue and aircraft pressurization, and de Havilland's engineers were innocent victims of this lack of knowledge. For instance, a the time of writing, the Wikipedia article on the Comet states that
According to de Havilland's chief test pilot John Cunningham ... representatives from American manufacturers such as Boeing and Douglas privately disclosed that if de Havilland had not experienced the Comet's pressurisation problems first, it would have happened to them.
However, at the time of the Comet's introduction in 1952, Boeing, Douglas, and Lockheed all had pressurized aircraft in operation. As the images linked below illustrate, they all had rounded or circular windows:
Boeing Stratocruiser (1949)
Douglas DC-6 (1947)
Lockheed Constellation (1943)
Most notably, the Boeing 367-80, Boeing's prototypical jet-powered airliner, which presumably cruised in conditions similar to that of the competing Comet, rolled out in May of 1954 with what appeared to be circular windows on its port side. This roll-out occurred well in advance of the October 1954 formation of the Cohen Committee which ultimately revealed square windows as the cause of the Comet crashes.
So, my question is, did American manufacturers choose rounded windows for their aircraft for reasons other than structural safety, or did they have access to knowledge about metal fatigue either unavailable to or disregarded by de Havilland which allowed them to design safer aircraft?