Short answers, pulled from the story.
Chainmail was created by Gary Gygax and Jeff Perren. Gygax expanded a four-page set of medieval rules written by Perren, a hobby-shop owner and fellow member of the Lake Geneva Tactical Studies Association, and Guidon Games published the result in 1971.
Henry Bodenstedt's Siege of Bodenburg, a medieval wargame designed for 40mm miniatures, first sparked Gygax's interest. Gygax encountered it at Gen Con I in 1968; its rules had been published in Strategy and Tactics magazine and were later used by Jeff Perren as the basis for his own medieval ruleset.
The Chainmail fantasy supplement was a 14-page addition to the first edition rulebook, introducing heroes, super-heroes, wizards, elves, orcs, dragons, and elementals. It divided creatures between Law and Chaos, drawing on the alignment philosophies of Poul Anderson and Michael Moorcock's Elric series, and referenced the fiction of J. R. R. Tolkien and Robert E. Howard.
Chainmail's fantasy supplement provided the conceptual and mechanical groundwork that fed into Dungeons and Dragons. Gary Gygax wrote in the June 1978 issue of The Dragon that early D&D players often used Chainmail rules as a foundation, and the 1976 supplement Swords and Spells was described by Tim Kask as the "grandson" of Chainmail.
Three editions of Chainmail were published. Guidon Games released the first edition in 1971 and the second in 1972. TSR later acquired rights to the Guidon Games back catalog and published the third edition starting in 1975, with eight printings through 1985.
The cover depicted a fighting crusader inspired by an illustration by Jack Coggins from his book The Fighting Man: An Illustrated History of the World's Greatest Fighting Forces. Don Lowry of Guidon Games drew the fantasy supplement cover illustration himself, basing it on a Pauline Baynes drawing made for Tolkien's Farmer Giles of Ham in 1949.