The B platform (also known as the B body) is a full-size rear-wheel drive car platform produced by General Motors from 1926 to 1996. Originally made for Oldsmobile and Buick, all of General Motors's five main makes would use it at some point. It was closely related to the original rear-wheel drive GM C and GM D platforms, and was used for convertibles, hardtops, coupes, sedans, and station wagons. With approximately 12,960,000 units built, divided across four marques, the 1965-70 B platform is the fourth best selling automobile platform in history after the Volkswagen Beetle, Ford Model T and the Lada Riva. Originally, the B platform was used for Buick and Oldsmobile products, with the first GM A platform for Chevrolet and Oakland, and the above GM C and D platforms devoted to Cadillac. During the General Motors Companion Make Program, Vikings and Marquettes were also manufactured on this platform, as were La Salles from 1936 to 1940. The B platform was used for the Pontiac Streamliner Torpedo and Streamliner, the Oldsmobile L-Series, Series 70 and Series 88, the Buick Special and Century, the LaSalle Series 50 and the Cadillac Series 60, Series 61 and Series 63. The B platform became GM's base model platform in 1958, when all existing Chevrolet products were upgraded to the B platform. For the 1959 model year, the previous A and B bodies were built on the new B platform that lasted until 1996. During this period, the B was the most modest of GM's three full-sized platforms, slotting below the upscale C and the luxury D. The A platform designation would be resurrected by GM in 1964 for a new series of intermediate-sized cars including the Chevrolet Chevelle, Pontiac Tempest, Oldsmobile Cutlass and Buick Skylark.
The GM B platform was introduced in 1926 with the Buick Master Six, and the Oldsmobile Model 30, and had at least 12 major re-engineering and restyling efforts, in 1937, 1939, 1941, 1949, 1954, 1957, 1959, 1961, 1965, 1971, 1977, and 1991; along with interim styling changes in 1942, 1969 and 1980 that included new sheetmetal and revised rooflines. The platform was downsized in length by approximately 10 inches in 1977 and reduced in weight by an average of 800 pounds. In 1991, the platform received its last major redesign, regaining several inches in length, numerous frame improvements and reinforcements, while the shorter wheelbase remained unchanged. The last B-cars rolled off the line in 1996, leaving only Ford producing domestic large rear wheel drive sedans until the line was phased out in late 2011, with Chrysler reentering the market with their LX platform in 2005.
Known for being durable and reliable, most B platform cars used suspensions utilizing coil springs in the front and leaf-springs in the rear until 1958, when they switched to coils in the rear; exceptions include the 1959-60 Oldsmobile 88, which used coil springs in front and multi-leaf springs in the rear. All B platform cars since 1965 have used perimeter frames with side rails, along with the 1961-64 B platform Pontiacs and Oldsmobiles. The 1958-60 Buicks and 1959-60 Oldsmobiles used a ladder-type frame while an X-frame without side rails was used on 1959-60 Pontiacs, 1959-64 Chevrolets and 1961-64 Buicks.
The Rear Drive B platform was the last platform design to have the gasoline tank filler port behind the license plate after 1958. Exceptions included all station wagons, as well as all 1961-64 cars - which had the tank filler in the rear fender on the driver's side and 1965 Buicks which had their fuel filler door above the license plate. Also, the Pontiac B platform vehicles from 1941 till 1948 had the fuel filler door in the rear fender on the driver's side. Very likely this would also have been the case with the B platform Oldsmobile and Buick.
By 1986, the Buick LeSabre and Oldsmobile Delta 88 moved to the GM H platform. And in 1991, Chairman Robert Stempel said:
"There are some corporate things we're going to drive; we are a front-drive corporation."
He also stated that the rear drive 1992 Buick Roadmaster and Chevrolet Caprice had been produced "over my dead body."  No successor for this platform was made after 1996, when the Roadmaster and Caprice ceased production. However, the Caprice would be revived and built on the Opel developed V platform for the Middle East but built by Holden, while years later the Caprice for police use along with the Impala SS successor, the Chevrolet SS, would return to America built on the GM Zeta Platform which was developed by Holden in Australia.