Technically a Transport

Transports. We all know them. Slow. Weaponless. Altogether,quite vulnerable. But what if you could fix that little issue? In comes this thing; A humble battlefield bus turned into a substitute front-line tank. Or gunship. And it can still drop off its passengers and cargo most of the time. But who'd want to?

Video Games

 * Battlefield 2142: With the Air Transports filled with various members of a squad. Most commonly being Engineers, a Medic/Soldier, and Supports.
 * Supreme Commander: A T1 or T2 Dropship loaded with T1 Light Assault Bots. The Cybrans can stick a T2 Mobile Stealth Field Generator with the LAB's to make it impossible to shoot down early without Omni-Sensors. In Forged Alliance,the UEF gets a T3 Dropship with a self-projected shield to jack survivability. The in-house term is "Ghetto Gunship"
 * Twisted Metal: Just look at half the vehicles, you'll see it.

Real Life

 * The AC-130 gunship. It's simply a Hercules cargo plane with guns and ammo rolled on board,pointing out the port side. The AC-47 and AC-119 having a similar history.
 * Maybe not "a near unstoppable force of destruction", but a technical is a pickup truck fitted with guns (Also seen in Command & Conquer Generals/Zero Hour, they upgrade increasing their strength through wreckage.)
 * During the American Civil War the Confederacy salvaged the USS Merrimack and outfitted it with iron plating, renaming it the CSS Virginia.

Tabletop Games

 * Car Wars. Big rig trucks are often armed with extensive weaponry in case they're attacked by bandits or have to engage in autodueling.
 * Star Fleet Battles. Many interstellar civilizations used armed transports and Q-ships to defend convoys from Orion pirates and other raiders.

Film

 * Star Wars. The Millennium Falcon. Originally a stock light freighter, but has since been upgraded with souped up drives, heavy-duty quad laser cannons, concussion missiles, and a sensor suite that's like seeing the future.
 * The Batmobiles seen in The Dark Knight Returns and Batman Begins.
 * In the Dawn of the Dead 2004 remake, the survivors turn a tour bus into an armoured zombie killing death machine (complete with slots that let them use chainsaws to get rid of undead hitchers) in order to make a break towards the pier.
 * The a Team with their modified vehicles.
 * The movie The Pentagon Wars is about a Real Life development project that went this way: the Bradley Fighting Vehicle started out as a pure troop transport vehicle, but as development continued it acquired more and more features of a tank.