Mimban was first created for the story "Splinter of the Minds Eye" that was published soon after the first Star Wars film was released. Splinter of the Minds Eye was commissioned by George Lucas that should the first film failed at the cinema then this story would form the basis of the next film.
The events of Splinter was set on a swamp world and only featured Luke Skywalker and his sister Princess Leia. Unlike the other films, the story featured one planet to cut down costs should it be made into a film. As we know, the film was a roaring success and therefore the story wasn't turned into anything.
The planet is swamp/muddy planet and in the film has a battle between the Imperial army and Separatist droids. Having failed to make the cut to be an Imperial Navy pilot, Han has become a foot soldier and is tasked with taking on the Separatists.
It is here that Han Solo first meets Chewbacca after being thrown into an underground cell with the wookie. Instead of being killed by the Wookie, Han Solo makes friends with the Wookie. Han gets taken off the planet by Tobias Beckett and becomes a smuggler.