The M103 heavy tank was intended to take on the Soviet IS-3 and T-10 series of heavy tanks, and used an elliptical hull and turret developed by Joseph Williams of the Ordnance Tank and Automotive Command. The first production M103s were not considered fit for service and were placed in storage until their faults could be rectified. Not until 98 modifications were applied to the vehicles, including fixes to ammunition, turret controls, and sighting and gun controls, were the tanks finally accepted and standardized in April 1956. The first 60 vehicles featured a small driver's hatch, while later tanks came equipped with a larger hatch. Not all of the small hatches were able to be fitted with the mount for the M24 infrared periscope. The commander and gunner sat in the bustle of the very large turret, and two loaders were required to feed the separate projectile and propellant to the potent 120mm gun. The commander's .50cal machine gun could be fired from under armor, but the commander had to expose himself to reload the weapon. The tank was 120.8125" (306.8638cm) tall to the top of the machine gun mount, and 115.25" (292.74cm) tall without the machine gun mount. One coaxial machine gun was placed on each side of the 120mm gun.

M103A1 introduced a turret basket into the series, eliminating the need for the loaders to dance around the tank whenever the turret traversed. The gunner was moved about four feet (120cm) forward, and he was provided with a new telescope as a secondary sight, which replaced the right-hand coaxial machine gun. The tank's rangefinder became the commander's responsibility and was coupled with the M14 ballistic computer. The turret traverse was changed to an electric amplidyne system, and the commander's cupola was modified, eliminating the geared traverse, and as well the ability to fire the .50cal machine gun from within the vehicle. The front of the 120mm gun's blast deflector was eliminated due to erosion around the deflector holes.

The improvements made to the tanks finally allowed the design to meet Continental Army Command standards, however by the time the new features were ready to be rolled out the Army had relegated the M103 to standby status and did not plan further production. The Marine Corps, on the other hand, approved conversion of 219 of their M103s to M103A1 standard in June 1957. This program was completed by July 1959, and in February of that year the Marines loaned the Army 72 of their M103A1s.

M103A2 development was initiated by the Marine Corps in June 1961, and the tank was powered by the diesel engine also seen on the M60 and M48A3 tanks. Like these tanks, the M103A2's rear hull was changed to incorporated an infrared-insulated rear deck, and the engine exhaust was routed through grille doors in the hull rear. The new rear hull necessitated a new gun travel lock, which replaced the tripod arrangement with a travel lock mounted on the extreme rear of the top deck. The diesel's better fuel economy and an increased load of fuel greatly increased the tank's range. The M103A2's fire control system incorporated metric units, a more user-friendly coincidence rangefinder was installed, and a xenon searchlight could be mounted. The three pilot M103A2s had the second and fifth track return rollers removed from each side.