The Making of Original D&D p72 wrote:
Outdoor Survival
Excerpts from Outdoor Survival; 1972
Avalon Hill published the game Outdoor Survival in September 1972, so it was quite new when Arneson mentioned it to Gygax in a 1972 letter to Gygax (page 76). Outdoor Survival incorporates a Wilderness Encounter system in which, when a wanderer enters an unexplored hex, a die is rolled to determine if a beneficial or harmful incident occurs. Arneson adapted this to the Blackmoor campaign. As he explained to Gygax, "the wilderness encounter aspect [of Outdoor Survival] was modified to mean that you met various creatures which you had to hack your way thru. There is also the chance that you will die of thirst or hunger (how ignoble) while wandering around lost."
Outdoor Survival also has a Life Level Index that may have influenced the idea of hit points. The Life Level Index grants players fifteen points of thirst and hunger damage they can withstand, based on tracking a Water Index and Food Index.
Arneson eventually built an "Encounter Matrix", a table of random monsters that could be found in each of the six types of squares present in Outdoor Survival: Open (or "clear"), river, mountain, desert, woods, and swamp. This matrix isn't included here, but it was reprinted in The First Fantasy Campaign. The creatures that populated the Encounter Matrix were effectively the same as those included in the second edition of Chainmail. These rules became the foundation for D&D's random encounter system and wandering monsters.