This little trick requires VTOL technology.
First of all go to vehicle design screen and select any body.
Select VTOL propulsion.
Select a VTOL weapon for example the Thermite Bomb bay.
Go back to the propulsion and select something other than VTOL.
You should now have VTOL weaponry on a ground based tank. A recommended test unit is the Thermite Bomb Bay Tiger Tracks. This does work trust me, I don't lie.