I was going to put up the new John Carter Trailer but instead found this! According to Wiki :
MGM and Bob Clampett production
In 1931, Looney Tunes director Bob Clampett approached Edgar Rice Burroughs to adapt A Princess of Mars into a feature length animated film. Burroughs responded enthusiastically to the idea, recognizing live action would have limits to where an adaptation could go visually, but advised Clampett to write an original adventure for Carter. Working with Burroughs' son John Coleman in 1935, Clampett used rotoscopeand hand-drawn techniques to capture the action, tracing over the motions of an athlete who performed John Carter's powerful movements in the reduced Martian gravity. Clampett designed Tharks, the Green Martians of Barsoom, which he attempted to give a believable appearance, and produced footage of them riding eight-legged thoats at a gallop, which showed all eight legs in coordinated motion. He also produced footage of a fleet of rocket ships emerging from a Martian volcano. MGM was to release the cartoons, and studio heads were enthusiastic about the series.
The test footage produced by 1936 received negative reactions from exhibitors across the US, especially in small towns, many of whom opined that the concept of an Earthman on Mars was too outlandish for Midwest American audiences. The series was not given the go-ahead, and Clampett was instead encouraged to produce an animated Tarzan series, an offer which he later declined. Clampett mused that there was irony in MGM's decision, as the Flash Gordon series released in the same year by Universal Studios was highly successful, and speculated that MGM thought that serials were only played to children during Saturday Matinees, and the John Carter tales would be seen by adults during the evening. The footage Clampett produced was for many years believed lost until Burroughs' grandson, Danton Burroughs, found some of the film tests in the Edgar Rice Burroughs Inc. archives in the early 1970s. Had A Princess of Mars been released, it may have beaten Walt Disney's Snow White and the Seven Dwarfs to become the first American feature-length animated film.
A Princess of Mars would have been the first American feature instead of Snow White!