|Mentzer Basic cover art prototype.|
Above is Larry Elmore's original idea for the cover art of Mentzer Basic. Frank Mentzer posted about it over on Facebook recently, with a photo of the original art taken by Jon Peterson. Here's part of what Frank wrote about it:
"Initial cover concepts were similar to the previous edition, portraying a group of adventurers with a dragon. That was almost used, but it bothered Gary [Gygax]. After the color images were ready to be finalized in oils, after a lot of plans were made, Gary changed course. He asked Larry to focus the action on just one fighter and one dragon ... and the world-famous Red Box cover emerged."
This version has more elements in common with the Holmes Basic cover art painted by David Sutherland. In addition to the huge red dragon on a pile of gold, there is a block stone archway in the background, and a wizard framing the left side of the composition, both casting a spell and providing an upraised light source (here a glowing staff, in Sutherland's a torch).
Larry Elmore is currently offering a limited edition print of this original concept via his website. Thanks to Gus Landt for bringing this offer to my attention via a post on the Acaeum. Note that above I cropped the blank margins off the original image to focus on the painting, but that the print being offered includes the margins, which have production notes.
Update: Here's a mock-up of the Mentzer Players Manual with this artwork that Frank posted in the comments to his Facebook post:
Update #2: Over on the Acaeum, misterspock posted an auction photo of a TSR Companion Rules box mock-up. IIRC a photo of this mock-up appeared in a TSR catalog prior to the release of the set. This cover has prototype art showing the same party of adventurers - wizard in red with staff, halfling in green, fighter with the same boots and shield.