Come to think of it, I think it was a recurring AD on the spar. Not sure though. Dang it, might have to call my friend who did the background research.
Lockheed virtually stopped production of the Electra line during WWII, probably to concentrate on other more important areas of war materials (Lightning, anyone?). That probably started it... By the end of the war, the Electra series was gone... obsolete so to speak, but why did the Twin Beech survive?
I think, it was due to sheer numbers. Beech was able to produce an enormous amount of aircraft that were still available and used as transports after the war. Also, not long after, Beech totally redesigned the airplane starting with the high cabin or "Super" 18s (E models). The airframe is versatile, tough, and is the most modified airplane in the world (200+ STCs).
Interesting notes...
The US Air Force retired their C-45 in 1963...
The US Navy in 1972...
The US Army in 1976...
Not bad for a bird designed in the 30s.