So it appears very few people know this - but HoverTank 3D was the first 3D first person shooter to be released by the company that would eventually be known as 'ID Software'.  Released in 1991 it used EGA polygons for the walls, and had very simple graphics.  I Always enjoyed this game - especially the little touch that the HoverTank always 'bobbed' as it moved, like a real hover tank would.  From the game files:

###                           About the game                         ###

In HOVERTANK 3-D you play mercenary Brick Sledge, hired by a beneficent
organization to rescue people from cities targeted for limited nuclear 

Get in, get 'em, and get out, before the missile hits.

Climb on board your new HOVERTANK and go to it!

###                      Summary of how to play                      ###

The cities are full of nuclear mutants, armored patrols, and other terrors
bent on your destruction.  You must enter the cities and rescue the
innocent humans needing your help.

Here's the catch --- the nuclear missiles are on their way.  You must
rescue the people... the ones that are still alive.  You and the
legendary HOVERTANK will not be teleported back to safety until you've
accomplished your mission.

You have an energy cannon as your main weapon.  Holding down the fire
button will build up power in the cannon.  Release the button to fire.

After all the people in the area are either rescued or killed, a warp
gate will appear.  And, it may not appear where you teleported in...

Other things to know...

Watch the time.  If that missile arrives, goodbye HOVERTANK.

Be careful about picking up the people.  You don't want them killed by
your stray cannon shots (and, of course, you get more money for more
safely rescued people.)

Good luck!

HoverTank 3D runs great under Virtual PC:

HoverTank 3D under Virtual PC   HoverTank 3D under Virtual PC   HoverTank 3D under Virtual PC   HoverTank 3D under Virtual PC

If you want to read a little more - you can check out ID's (admittedly minimal) website on this game: