I'm surprised I can't find a thread on this yet. So Larry Elmore's original submission for the cover of the classic Red Box Set is now available as a print from Larry Elmore's website:

Larry Elmore · D&D - Other Basic Ancient Red

Even better, it's 20% off (as are all his prints) until the end of the month!

So, all these years I thought that guy was taking on that big red alone.  Turns out he DID have some backup after all.

Seems less heroic now, though.  

his original concept for the Companion Set art also had a party of adventurers

I assume there was an expert set concept that was along the same lines. I have not seen one. The next time I talk to Larry, I'll ask if he remembers. If so, maybe I'll see if I can get him to do a rough sketch of what that looked like.

Yes, this print of the Companion set concept art was from an auction about two and a half years ago.

edit: Thanks for that update post on your blog, looks good! just wanted to say that yes, this artwork did appear in one of the TSR catalogs for certain.

It just dawned on me that the concept art for the Companion cover could have actually been for the Expert set. They may have needed some art real fast when the Companion set was in development and just re-purposed that art for the catalog and the Gen Con booth display print. They re-purposed a lot of art during that time.
The Hydra they are fighting is a monster introduced in the Expert set, maybe that's why Larry picked it for the cover concept.

sure, that would be great to have it collected on one page

here's another photo of that Companion concept art print

I like the concept art/mock-ups better than the final choices.