According to a caption in the book The Jet Age by Robert Serling, the triangular design was to maximize the passenger’s view downward while minimizing stress on the fuselage around the window.

Great Book! Fantastic story and pictures about the Pan Am 707 that landed near San Fransisco with 3 engines and 20 foot of it’s outer wing missing!

PAA 704, The Pilot was Captian Kime. WE picked up parts all over South City that evening. The wing came down between two trees that supported it on its side. The Engine came down in a machine Shop.Boeing came to Travis and rebuilt the Aircraft.” Eugene Hollingsworth.