A rare, and enormous, 1950 General Motors Futurliner bus sold at Barrett-Jackson's Scottsdale auction Saturday for $4 million.

The money will benefit a charity for military members and their families.

The bus sold as part of the automotive collection of Arizona-based real estate developer Ron Pratte. He bought the bus in 2006, also at a Barrett-Jackson Scottsdale auction, for roughly the same amount.

The bus was one of 12 built by GM (GM) to lead its Parade of Progress tours in the 1940s and '50s. Starting shortly after the 1939 New York World's Fair, these events showed off General Motors products and concept cars at events around the nation.

Not long after they got going, the events were stopped for World War II. Years later, the tours started up again and the buses underwent major modifications.

Despite their enormous size, these buses could carry only three occupants including the driver. The sides opened outward to reveal a stage.

The buses were originally powered by two-stroke diesel engines, and after 1953 they were fitted with 145 horsepower six-cylinder gasoline engines.

The driver's seat was 10 feet off the ground. Parade organizers had to carefully plan routes to avoid low bridges or other obstacles. Eight tires held its 30,000 pound weight.

The tours were permanently shut down in 1956, and most of the buses were modified for other uses.

Today, nine of the vehicles survive and only three -- this one included -- have been restored and returned to their original Parade of Progress configuration.

Proceeds from the sale of the Futurliner bus are being given to the Armed Forces Foundation, a nonprofit that serves military service members, veterans and their families.

Generally, cars sold for charity tend to fetch higher prices, and the Futurliner probably would have otherwise sold for about $3 million, said McKeel Hagerty, CEO of the collector car auction company Hagerty Insurance.

At the same auction, Pratte also offered for sale his 800 horsepower 1966 Shelby Cobra "Super Snake." Pratte had bought the car for $5.5 million at a Barrett-Jackson auction in 2007.

This time, bidding for the Super Snake went to $4.7 million. Pratte had set a minimum selling price, or reserve, higher than that, so the car was not sold at auction. But it did sell after the auction, in a private transaction, for $5.1 million.

The Super Snake was one of only two like it ever built. This one was owned and driven by Shelby American founder Carroll Shelby.

The other was built for comedian Bill Cosby who had requested the car after seeing Shelby's.

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The Super Snake had a roaring engine that, even at idle, sounded like an artillery attack. With 800 horsepower, in an era before computerized driver aids like electronic stability control, this Cobra needed to be handled with extraordinary caution.

The car later became part of a famous Cosby comedy routine.

Cosby only drove the car once, then sold it because he was so terrified of it. Apparently, he was right to be intimidated. That car's subsequent owner drove it off a cliff into the Pacific Ocean and died.

That left this car, Carroll Shelby's original, as the only remaining Super Snake.

Even ordinary "big block" Shelby Cobras of this era are valuable cars worth over $1 million in good condition. This one, with its heavy-hitting horsepower and connection to the legendary Carroll Shelby himself, easily outpaced those more ordinary cars on the auction block even as it failed to sell.

Another one of Pratte's collection sold at the auction was a 1954 Pontiac Bonneville concept car that got $3.3 million.

The first GM car to carry the Bonneville name, the car marked the beginning of Pontiac's performance image. Two were built, a bronze one and this turquoise colored car. Pratte had bought this car at the same Barrett-Jackson event at which he bought the Futurliner. At that time, in 2006, he paid $2.8 million for it.

Correction: An earlier version of this article incorrectly reported the sale price of the GM Futurliner. It sold for $4 million, not $5 million.