Via Starwars.com:

THE BEHIND-THE-SCENES STORY OF THE FASTEST HUNK OF JUNK IN THE GALAXY.
Welcome to From Concept to Screen, an ongoing series about the various stages of the production that your favorite character, vehicle, creature, location, or scene of the Star Wars saga had to undergo before arriving on the silver screen.

This time we take a close look to the ship that everybody wants to fly and returns in Star Wars: The Force Awakens this month: the Millennium Falcon. It turns out that the real-life history of this ship is just as rich as its movie counterpart.

Concept

The first time that a “pirate ship” for Han Solo is mentioned in any of the scripts was in the second draft, dated January 1975. In this draft Lucas had written a scene in which Han was serving onboard a pirate ship along with fellow crewmates Chewbacca, Montross Holdaack (a character who has been in almost every version of the story, yet still never made it to the screen), and Jabba the Hutt, who at that point was still a human. Han was able to steal the ship away from its captain by faking a reactor overload, causing everybody but him, Chewbacca and Montross to flee the ship. A later outline would remove this scene, making Han Solo the owner of this pirate ship from the start of the story. The revised fourth draft, dated March 1976, first named the ship as the Millennium Falcon.

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