National Hamburger Day is May 28. McDonald's is the most famous fast-food chain, but you might not know these facts about its burgers. The first "Big Mac" sold for 45 cents and was named by a 21-year-old secretary. The first McDonald's restaurant before it became the chain it is today wasn't primarily a burger restaurant. ray kroc Founder and chairman of McDonald's Corporation, Ray Kroc, stands outside one of his franchises, holding a hamburger and a drink.Bettman/Getty Images In 1954, future McDonald's Corporation founder Ray Kroc, a Multimixer milkshake machine salesman, came across a small restaurant in San Bernardino, California, run by the McDonald brothers. The very first McDonald's Bar-B-Q menu was large and its most famous offering was hot dogs. However, in 1948, the McDonald brothers switched the concept to become a burger restaurant and reduced the restaurant's offerings to just nine items: hamburgers, cheeseburgers, soft drinks, milk, coffee, potato chips, and a slice of pie. In 1949, potato chips were swapped with french fries and McDonald's began selling milkshakes. Due to the restaurant's self-service counter, there was no need for waiters or waitresses. Burgers were cooked ahead of time, wrapped in paper, and kept warm under a heat lamp, which meant customers received their food in record time. Amazed by the efficiency of the restaurant, Kroc wanted in. He became the first McDonald's franchisee agent. In 1948, you could get a McDonald's burger for just 15 cents. mcdonalds old On the sign is "Speedee" McDonald's original chef logo.David McNew/Getty Images At the original McDonald's locations, before Ray Kroc revolutionized the company and founded what we know as McDonald's today in 1955, a burger cost just 15 cents. McDonald's has reportedly sold hundreds of billions of burgers during its years in operation. mcdonald's employee burgers A McDonald's employee preparing burgers.Nir Elias / Reuters From the beginning, McDonald's has touted its burger sales in its advertising. In 1955, McDonald's claimed that it had sold over 15 million burgers — this was just seven years after the McDonald brothers closed their barbecue stand and reopened it as a burger joint. Today, it's estimated that McDonald's has served billions and billions of burgers, with the Wall Street Journal estimating the chain sold its 300 billionth burger around 2013.