Somebody on Twitter suggested that every time someone says, "This thing writes itself," a writer's pencil breaks. Which he insisted was no big deal, but I immediately felt his (minor) pain. He went on to suggest that in addition to the point of his pencil, the affected writer would also lose the use of his wings. Which they don't tend to use that much anyway.
So, writers are like angels with limited flight capability. But it was a writer who first came up with that bit about every time a bell rings an angel gets its wings. It was spoken by George Bailey's daughter Zuzu, but it had to come from somewhere. Little girls in moving pictures don't just make up clever things like that to say, magically delivering on the premise of the angel that kept dad from committing suicide on Christmas Eve. If you need to know the name of that little girl who played the role of Zuza, she was Karolyn Grimes, but those were not her words. Originating as a twenty-four page pamphlet in 1943 entitled "The Greatest Gift" written by Philip Van Doren, the screenplay for the Wonderful Life that George Bailey ends up being happy to have lived in spite of all rough spots went through many hands and plenty of revisions.
Exactly where did the idea that angels had to earn their wings show up? It might have been that Frank Capra, who directed the film and had plenty of input on the screenplay might have been responsible. Or maybe it was wedged into one of the early drafts from Dalton Trumbo, Clifford Odets, or Marc Connelly. Perhaps it was one of the next group that included Frances Goodrich and Albert Hackett, Jo Swerling, and Michael Wilson who figures there should be some sort of merit based system by which departed spirits might become more avian. Or perhaps that's just the kind of thing that would have originated from the pithy pen of Dorothy Parker, who was brought in to do some polish work on several versions of the script.
Eventually, all of these folks were forced through the unseemly process of Screenwriter's Guild arbitration. What came out of the back end of that sausage grinder was a screenplay credited to Goodrich, Hackett, and Capra, with "additional scenes" by Jo Swerling.
So who got their wings? Who got theirs clipped? There's little doubt that many pencils were broken in the process. Many of these were most likely brought on by some smart aleck insisting that such a lovely little story probably just wrote itself.