George Lucas actually developed the idea for Raiders of the Lost Ark before Star Wars. He wound up shelving it though so that he could work on his little outer space adventure instead. When that became a phenomenon Lucas had enormous clout in Hollywood and was able to cut a sweet deal with Paramount for Raiders that gave him something like 40% ownership of the film and a hefty percentage of the profits.
He was also able to get his buddy Steven Spielberg to direct, something that was key to the films ultimate success. Lucas wanted to call his character "Indiana Smith" and it was Spielberg who told him the name didn't work. Lucas then suggested "Jones" and the rest is history. Released in June 1981 it became the highest grossing film of that year and, when adjusted for inflation, is still one of the highest grossing films of all time.
Several actors tried out for the lead role. Spielberg wanted Harrison Ford but George Lucas didn't want Ford's name to become linked too closely with his own. Subsequently, Tom Sellek was offered the role but was unable to take it due to scheduling conflicts with "Magnum P.I.". Jeff Bridges also reportedly turned the role down. With just three weeks to go before shooting was to start Steven Spielberg finally convinced Lucas to give the role to Ford.
It seems crazy now to think of anyone else playing Indiana Jones. Ford seems to have been born for it. It's a tribute to him that he's probably just as well known now for playing Indiana Jones as he is for playing Han Solo.
Watch the original trailer here.