The X-Model Robots were a series of robots created by scientists Abel Stack, Oliver Broadhurst and William Haines as part of a top-secret U.S. Army project. Their goal was to create a sophisticated weapons system capable of independent thought. Dr. Stack took X-51, one of the the projects robots, home to test his radical theory that a robot could only function like a human if it was raised like one. The other 50 models began displaying psychotic tendencies due to insufficient programming. The project was determined a failure and the order came down to destroy them. Each of the robots was equipped with an auto-destruct device that could be triggered remotely. Stack managed to remove the self-destruct mechanism from X-51, but figured he couldn't dispose of the bomb before it exploded, and urged Aaron to leave him. X-51 vowed that Stack's dream of human and robot peace would come true.
During the A.I. Army's robot revolution, the remains of the destroyed X-Model Robots were bought by Baintronics from the U.S. Government, and each model was rebuilt using X-51 as a basis. Having determined that free thought was the robots' flaw that made them defective, Arno Stark deprived them of free will, equipping them with a device that transmitted a brainwashing code into their systems.
When the A.I. Army attacked Bain Tower, Arno used Jocasta to lure Machine Man, who was one of the group's key members, into a Baintronics base in New Jersey where he encountered the rebuilt X-Model Robots. While they were attacking Machine Man, he deduced the existence of this device and graced X-48's chassis with his chainsaw, exposing his insides to find it. Machine Man removed the device from X-48, causing him to instantly have a mental breakdown and attack another of the X-Models when it tried to calm him down. Machine Man eventually disabled Stark's transmission in all the X-Models, which allowed him to go around them and continue looking for Jocasta while a group of Bain-Bots arrived to subdue his predecessors.