Between the first attempt at an adaptation in 1936 and the one finally released in 2012, John Carter knew quite a few false starts. Juste before Disney bought the rights again, those were at Paramount. Robert Rodriguez was supposed to direct the movie in the beginning. A friend of Harry Knowles, the founder of the famous Aint'It Cool News website, and knowing about his passion for "A Princess of Mars", he made him hired as a consultant/co-producer. Rodriguez left the project... because of Sin City. He wanted Frank Miller to be credited as co-director on this movie, which wasn't allowed by the Director's Guild of America. So he was forced to resign from it, and Paramount couldn't therefore hire him anymore for their project. Then enters Kerry Conran, director of Sky Captain and the World of Tomorrow. He would work on the movie for a few months before leaving for unknown reasons. What's left of this period of time is this demo reel, that Harry Knowles posted on July the 3rd of 2015 on Youtube in high definition:
It's superb, even if some sets way to fantasy-looking would have probably looked too fake for my taste.
Fun bit of trivia: it was edited by Erk Jessen, who ended up working on John Carter too!