A Princess of Mars

"In 1931, animation legend Robert Clampett approached Burroughs himself with the idea of making the book into an animated film, to which Burroughs was enthusiastic. The author's son, John Coleman Burroughs, helped Clampett create an extensive array of sketches, sculptures and production notes while the rights to the project were picked up by MGM. However, Clampett and the two Burroughs soon clashed with the studio over the direction to take the film - the creators wanting to make a serious sci-fi drama, the studio wanting a slapstick comedy with a swashbuckling hero. Eventually, the studio pulled the plug on the entire project. Originally planned for a 1932 release, it would have been the first feature-length animated film (the honour of which is held by Disney's Snow White and the Seven Dwarfs (1937)). When Clampett toured and lectured at universities in the 1970s, he would often screen some of the uncompleted animation footage for enthusiastic audiences."
(From John Carter of Mars movie fansite.)
Pixar bought the rights last year. The movie is currently slated for a 2012 release.
Anyway, can you imagine how the history of animation would have been different if Disney's first animated feature was a sci-fi adventure rather than a fairy tale? Which reminds me-- the first piloted robotic giant war machine was invented by Jules Verne in "The Steam House." They show up again in The War of The Worlds. (The tripods.)