[exDM69]

That's the first thought that springs to mind, but not necessarily the case.

Aircraft from that era were quite solid and they did not have a pressurized hull. The Lockheed Electra and other similar aircraft from the same era flew in wartime operations and regularly came back full of holes.

A single window broken cover should not bring an otherwise sound 1930s era aircraft down.

[snowballsteve]

Exactly correct. There is no way a missing window would bring down the plane, especially the back window, unless by some freak accident the patch damaged a control surface on the tail by breaking free. One can think of an almost limitless list of more likely hypotheses.

[davvolun]

To be completely fair, a list such as insulating foam peeling off and striking the wing (http://en.wikipedia.org/wiki/Space_Shuttle_Columbia#Final_mi...) or a random bird striking a plane and causing a failure (http://en.wikipedia.org/wiki/Bird_strike#Incidents)? I'm not saying it happened, or even that it's likely, I'm saying stranger things have happened.