Windows NT 3.1 build 127

Windows NT 3.1 build 1.127 is the earliest known build of Windows NT 3.1. It was mentioned in an internal document from the Windows NT 4.0 source code, which was leaked in 2004. The document suggests this build already had a working GUI and was compiled before 1 May 1991.

According to Comes vs Microsoft plaintiff exhibit 427, the GDI code (for rendering UI) was not scheduled to be fully implemented until June 1991, so the GUI may only be partially complete in this build.