Your Mother Was a Martian

These rules are strictly fantasy Those wargamers who lack imagination, those who don't care for Burroughs' Martian adventures where John Carter is groping through black pits, who feel no thrill upon reading Howard's Conan saga, who do not enjoy the de Camp & Pratt fantasies or Fritz Leiber's Fafhrd and the Gray Mouser putting their swords against evil sorceries will not be likely to find DUNGEONS & DRAGONS to their taste.

The seminal influence of Robert E. Howard and Fritz Leiber on the creation of Dungeons & Dragons is well established, I think. The role of L. Sprague de Camp and Fletcher Pratt is probably less known, given how few people have even heard of, let alone read, the Harold Shea series. Even less known, I think, is the influence of the Barsoom stories of Edgar Rice Burroughs. And yet he's the very first author whom Gary Gygax mentions in the "forward" [sic] to Volume 1 of original D&D. 

Consider, too, Gygax's words in the (again misspelled) "forward" to Warriors of Mars, written less than a year later.

Worlds of heroic fantasy are many, but perhaps the best known of them all is the Barsoom of Edgar Rice Burroughs, where John Carter, Tars Tarkas, Dejah Thoris, etal [sic] adventure endlessly in eternal youth.

I don't think there can be any question that Gygax highly esteemed the Barsoom stories, which are included even in Appendix N (though, it should be noted, Burroughs is not listed among "the most immediate influences" upon AD&D). 

OD&D contains multiple references to Mars, such as the tables for wilderness wandering monsters in Volume 3. The column for "Desert" has a parenthetical note "(Mars)," with entries for Red, Black, Yellow, and White Martians, as well as for Tharks. There's also an "Optional Arid Plains" column with entries for Apts, Banths, Thoats, Calots, White Apes, Orluks, Siths, Darseen, and Banths. Now, none of these beings or creatures are given any game stats and indeed it wouldn't be until the 1981 Moldvay Basic Rules that this would change, when one of these – the white ape, albeit with only two arms – finally appeared in print. Additionally, Mars is cited as an example of another world where one might set D&D adventures.

As it turns out, Gygax did just that. One of his son Ernie's characters was called Erac's Cousin and had an adventure on what is quite clearly the Mars of John Carter. One retelling of his exploits can be found here, from which I quote the following:

One of Erac's Cousin's more memorable adventures occurred after he spotted a strange red star in the night sky. He drifted off to sleep thinking of the strange star and when he awoke he discovered he had been transported to Mars. To his surprise he arrived stark naked. Soon after his arrival, the mage was attacked by the Cannibals of Ugor. Much to his dismay, he discovered that magic didn’t work there, and he was forced to fight toe-to-toe with the bloodthirsty cannibals using nothing more than a tree branch. In time the unnamed adventurer adapted and ultimately excelled in is new environment. Due to the planet's low gravity the marooned wizard's strength was heroic. He could leap 20 to 40 feet into the air, and much further than that forward. During the many months that he spent there, being unable to use magic, Erac's Cousin began training as a fighter. Instead of using magic to defeat his enemies, he would now cut them down with a sword. Before returning to Oerth he had slaughtered hoards of Green Martians, and organized an escape from the mines of the Yellow Martians. Finally he discovered a method of returning to Greyhawk. He found Oerth in the night sky before going to sleep and when he awoke he was back home. Unfortunately his arrival home was similar to his arrival on Mars; naked. He had left a fortune behind on the red planet.

Erac's Cousin's awakening on Mars naked recapitulates Carter's own experiences and, if the reference to multiple colors of Martians were not enough of a giveaway, there are the Cannibals of U-Gor, which appeared in the 1930 story, A Fighting Man of Mars. Issue #3 of the first volume of The Strategic Review (Autumn 1975) features an article on randomly generating ruined Martian cities by James M. Ward. It's not specifically associated with OD&D, but it's another example of Barsoomian content in a TSR product. 

I think it is unquestionable that the fantasy genre as we understand it today – and hence the roleplaying games that derive from it – owes its existence largely to Edgar Rice Burroughs's stories of Barsoom, which even a youthful H.P. Lovecraft regarded highly (he would distance himself from them later in life) and which inspired generations of imitators and pasticheurs, including such luminaries as Robert E. Howard and Michael Moorcock. That Gygax, give his age and fondness for pulp literature, would have likewise admired and drawn upon these same stories should surprise no one. Nevertheless, I think the influence of Barsoom on D&D's development is underappreciated and ought to be known more widely.