Descent was the first truly 3D FPS. The original quake was released after, and it wasn't 3d, it was just like DOOM. 2D designed to make you think it was 3d.
Quake II was finally 3D, as it ran with Glide on the Voodoo series accelerators from 3DFX.
Game History... I'm such a dork.
Actually, they did release something for the original Quake to use the 3DFX chipset, but it was quite a time after RTM.