The M60 is an American second-generation main battle tank (MBT). It was officially standardized as the Tank, Combat, Full Tracked: 105-mm Gun, M60 in March 1959. Although developed from the M48 Patton, the M60 tank series was never officially christened as a Patton tank. It has been called a "product-improved descendant" of the Patton tank's design. The design similarities are evident comparing the original version of the M60 and the M48A2. The United States fully committed to the MBT doctrine in 1963, when the Marine Corps retired the last (M103) heavy tank battalion. The M60 tank series became the American primary main battle tank during the Cold War, reaching a production total of 15,000 M60s. Hull production ended in 1983, but 5,400 older models were converted to the M60A3 variant ending in 1990.