Legend has it the Millenium Falcon was redesigned and re-built at the very last last minute.
The original design of our favorite pirate ship was to be a longer more traditional ship. You might recognize the cone shaped cockpit and the radar dish.
But just before principal photography was set to start, Lucas got a whiff of the Eagle spacecraft from the television show Space 1999 and he felt (rightly so) that a ship as important to the story as the Millennium Falcon needed to stand on its own visual aesthetic.
So this original design underwent a minor facelift by changing the cockpit to a "hammer-head" orientation and a significant boost in scale. Thus, the original Falcon became the Tanitive IV, Rebel Blockade Runner, the first ship we ever saw in the Star Wars universe.
For the over-all look of the Millennium Falcon we know and love, the myth says that George Lucas was inspired by a hamburger with an olive next to it. Much like the mythic family dog origin of Chewbacca, I'm skeptical of that story. I suspect the same George that gave us Crystal Skull said, "Make it look like a flying saucer!" A sketch of what would have been the Blockade runner makes be think George was somewhat stuck in a 50's sci-fi thinking.
Thankfully, the boys at ILM used their genius and in a few short weeks, incorporated that saucer into something truly remarkable. As construction of the interior cockpit set based on the original Falcon design had already began, they retained the cockpit as well as the radar dish, and the over/under gun turrets used by Luke and Han. The result is an iconic ship that's "got it where it counts."
It is fascinating to see how these random decisions, these "oh crap" set backs and totally unexpected outside forces (or FORCE) can align to create things that inspire people, shape lives and make careers. So much of the original Star Wars movies works because Lucas was not able to complete his full vision. The limitations created greatness. His genius was focused through a limited lens of reality. Years later, we found that with the advent of unlimited CGI and without those physical limitations, his original vision probably would have been less inspiring.
Which Falcon do you prefer?