The Build Engine History
The Build engine was written by Ken Silverman in 1994 and has gone through several major enhancements from its initial version. Ken wrote a game named "Ken's Labyrinth" in 1992 which he sent demos of to several games companies. One of those companies was Apogee Software. Apogee wasn't interested in the game but they were interested in the engine. He later started writing a demo named "Build" in 1993 which he also sent out to several companies. Apogee offered him a contract to write the Build engine for them.
Ken has a page on his website which features a timeline outlining the development and events surrounding the Build engine. Ken also has available for download old demos of the engine at various points in it's development and now the full source code!
Source Code Releases
On the 20th of June 2000, Ken Silverman released the source code to the Build Engine. The code and related information is available on Ken's Build source page.
On the 1st of April 2003, 3D Realms released the source code to Duke Nukem 3D. The original announcement is on 3D Realms' site.
On the 1st of April 2005, 3D Realms released the source code to Shadow Warrior. The original announcement is on 3D Realms' site.