Watcom C 386 8.5a
Released in 1991 by Watcom
For DOS, Windows 3.x, and OS/2

Watcom C/386 8.5 was the first compiler to ship with a royalty-free 
32-bit DOS extender. It also included an extender for Microsoft 
Windows 3.0 that enabled the use of 386 32-bit code in 16-bit Windows 
3.x executables.

Much more information about this version can be found here: 
http://www.os2museum.com/wp/watcom-win386/