Disney's Snow White and the Seven Dwarfs was the first full-length cel-animated feature to appear on the big screen. But if things had gone a little differently, that honor might have gone to John Carter of Mars, which MGM was developing with legendary animator Bob Clampett. We do, however, have some remains of the failed project in the form of Clampett's test animation.
Clampett, who is known for his work on Looney Tunes and Merrie Melodies, as well as his television shows Time for Beany and Beany and Cecil, approached Edgar Rice Burroughs, author of the John Carter stories, about doing an animated adaptation of his fantastical space tales. Burroughs was so excited about the idea of an animated John Carter that he contacted MGM, who was making a mint on Burroughs' Tarzan films, about buying a series of animated shorts. Clampett went ahead and did a test animation sequence, which both MGM and Burroughs loved.
Sadly, the film was not to be. MGM's midwestern and southern sales representatives deemed John Carter too weird for their markets. And the world didn't just lose out on a John Carter movie; if the film had proved a success, more studios might have commissioned films with Burroughs' brand of strange scifi and fantasy visuals. There's a whole alternate film history we might have experienced.
Read Jim Korkis' article about the 1936 John Carter film, as well as his piece on other John Carters that never saw the projection light of day at http://www.johncolemanburroughs.com/0934.html and http://io9.com/5874427/the-disney-john-carters-that-never-were