Answer: Fuselage Stress

The earliest airplanes had square windows, a natural continuation of the familiar square shape found in trains, automobiles, and, of course, homes and offices. The trend of square windows in aircraft held steady through the early to mid-20th century. So why are aircraft windows round today?

The change came about as a result of the rise of high-altitude jet travel in the 1950s. High-altitude travel meant the cabin had to be pressurized for the safety of the occupants. Pressurizing the cabin caused micro-stress in the aluminum skin of the aircraft, and repeat pressurization and depressurization of the aircraft over time revealed a very nasty design flaw. The corners of the square windows created areas of concentrated stress in the aluminum skin of the aircraft, which lead to increased structural failure.

After a series of catastrophic airplane failures in 1954, wherein the fuselage of the airplanes failed at the corners of the windows, square windows were abandoned in favor of round windows. While the fuselage is still stressed by expansion and contraction, with round windows the stress in the metal flows more smoothly around the edges of the windows, helping to minimize the risk of catastrophic failure.