April 1997: Quake 2 engine purchase announced. (they got it in mid-late 1997 before Quake 2 was released). Scott Miller states that he is confident that DNF will be released by mid 1998 and prey in late 1998 making 1998 a bit year for 3DR. He also mentions that his confidence is not misplaced.
August and September 1997: First screenshots in PC gamer (hummer, farmer i think).
May 1998: First DNF E3 video shown. You can see that a lot of work has been done.
Late 1998: 3DR announces switch to Unreal tech. It is expected to take no longer than 6 weeks and 3DRealms says "DNF is a 1999 game."
Mid 1999: 3DR admits that the engine switch took a lot more time than originally estimated.
Late 1999: 3DR announces upgrade to UT level tech. It is expected to take two weeks. 3DR releases a Christmas card saying that DNF will be released in 2000.
Unknown 2001: Broussard states that DNF will definitely be out before Unreal 2.
Unknown 2002: Broussard states that DNF will definitely be out before DooM 3.