Here's a more detailed account of how DNF became vaporware and then was resurrected:
E3 1998 trailer, using Quake 2 engine:
E3 2001 trailer, using Unreal engine:
^^ this one I burned on CD-R over 10 years ago! The question is: was that only a machinima to fool E3 press? Is there a tech-demo of it, preferably leaked? I wanted to play that damned thing 10 years ago and I still do now!
I don' think there were beta leaks, but according to Joe Siegler (I think he had posted something at the official 3D Realms Forums although I probably won't be able to find the exact post right now) the game was quite playable and at some point, even, nearly finished (although I'm not sure if that one was the 2001 version), so the delays and the eventual company change was due to some other reasons. I don't know the exact story, but I think bits of it can be found in Joe Siegler's blog: