The original story was Night Bus by Samuel Hopkins Adams.  MGM bought it for Robert Montgomery but decided to trade it, along with their contract player Clark Gable, to tiny Columbia, in exchange for Capra to later direct a picture for them (Soviet, never actually made).

Gable had been complaining about the parts he was getting a MGM and so to teach him a lesson, Louis B. Mayer loaned him out to the “poverty row studio” Columbia.

Even more reluctant to make the film was the leading lady, Claudette Colbert, who upon completion of shooting confided to a friend that she’d “just finished the worst picture in the world”.

When Gable took off his shirt – in the famous ‘Walls of Jericho’ scene – and revealed a bare chest, sales of men’s undershirts plummeted as a direct result.

The sight of Gable eating a carrot in the film allegedly inspired the animator Bob Clampett to create Bugs Bunny.