In the beginning, planes had square windows. But in 1953 two plane crashes occurred and the square shape of the windows was found to be the reason for both. Where there is a corner, there is a weak spot. So, square windows having 4 corners have 4 weak spots, making them highly susceptible to cracking under stress. And actually the air pressure difference that the windows experience from inside to outside is enough of a stress. With this in mind, engineers decided to round the edges of the windows to evenly distribute the pressure and stress.