The D&D Alumni series has a great article up called An Elementary Look at the Planes.
The article goes in depth about the history of the planes, from it’s meager beginnings in Dungeon Magazine in 1974, to D&D fifth edition.
“Air, earth, fire, and water elementals are as old as the D&D game, appearing in the original Dungeons & Dragons rules (1974). However, these elementals only became part of something larger when Gary Gygax introduced a cosmology for Dungeons & Dragons in Dragon magazine. In issue 8 (July 1977), he laid out an entire “Great Wheel” that included “the ultra-pure Elemental Planes of air, fire, earth and water.” Those four “Inner Planes” existed alongside the material plane and the almost-elemental positive and negative material planes. Gygax didn’t provide many details, but it was a first look at a cosmic conception.”
Read the full article by Shannon Appelcline here.