As far as I can tell, there is no significant concept development of the Millenium Falcon between the intial blockade runner design and the subsequent 'real' Falcon. One moment it's the blockade runner, the next it's the falcon as we know it.

For a design departure that radical, you'd think there had been some sketches or mockups of some sort, right? The roughest, earliest drawings I've been able to find, are the thumbnails McQuarrie did for his revised Docking Bay 94 painting in A Gallery of Imagination, and clearly those were almost fully fledged designs!

So where did it come from? Am I just missing something obvious? (And no, it wasn't inspired by a burger.)