Black Gate got a great scoop on the origin of Mystara, the default fantasy campaign setting for basic D&D.
Lawrence Schick says:
In early ’74 Tom [Moldvay] came back from an SF convention with Dungeons & Dragons in its original white box edition. He DMed a session, I DMed a session, and suddenly we knew what we were going to create together: a fantasy world setting for D&D.Read the full text here:
We were both nuts about Clark Ashton Smith, Tom was a Michael Moorcock and Philip José Farmer fanatic, while I could quote chapter and verse from the works of Jack Vance and Fritz Leiber. So we knew what we wanted to create: a single world setting that would enable us to simulate the fictional realities of these, our favorite authors.