1. It was invented by a Pittsburgh-area franchise owner
In 1967, a small-time franchisee named Jim "MJ" Delligatti created a cheeseburger consisting of two 1.6oz beef patties, iceberg lettuce, pickles, American cheese, chopped onions, a special "Mac Sauce," and a double-cut sesame seed bun. At first, the burger was only available at three Pittsburgh McD's locations, but thanks to founder Ray Kroc’s enthusiasm, it went nationwide in 1968.
2. The middle bun was purely functional
As per Delligatti's original design, the Big Mac's center bun is used primarily to soak up the special sauce, keeping the sandwich from becoming a Big Mess.