I was working this weekend on a recently transcribed interview of Dick Huemer by John Culhane, which contains quite a few answers about Dumbo's origins and the famous Roll-A-Book that Michael Barrier discussed lat week in his great essay. Here is a quick excerpt:

[Some work had been done on [the story before Joe Grant and I took over]. Otto Englander may have done some work on it, and Webb Smith. They had done some preliminary work on it based on [...] the basic plot. And then Joe and I took it, and added things like the drunken elephant stuff, the pink elephant sequence, and the crows. We took it and we filled up the segments.]

(Thanks to Dan Caylor for the transcription)