From Greg Braxton in The Los Angeles Times:
The series was inspired by concerns [series creator] Vince Gilligan and writer friend Tom Schnauz had after The X-Files ended about the scarcity of screenwriting jobs. A casual conversation about what to do next prompted a joke about putting a meth lab in the back of a recreational vehicle and driving around the Southwest. That humorous and innocuous remark stuck with Gilligan and eventually grew into the premise for Breaking Bad (or, as it suggests, "raising hell"). Schnauz is now one of the writers of the show.