Prior to the wet-on-wet oil painting technique, Bob Ross was a "traditional" painter and painted oils the old-fashioned way that you see the classics painted. Instead of big brushes that can create scenes very quickly, you had to wait for layers to dry before continuing. Therefore, a simple painting could take several weeks to complete.

The wet-on-wet painting technique changed that, and he saw The Magic of Oil Painting, a program that would win an Emmy. Bill Alexander, the host of the program, grew up in Prussian Empire and was later drafted into WWII by the Nazis. When he was captured, he painted pictures of Allies' wives, and he and his wife immigrated to North America, were he revived the wet-on-wet technique.

The video located below from YT user electrotherapy is an episode featuring Alexander in that signature style of painting from The Magic of Oil Painting (an episode from at least thirty years ago). You can find many more on his account.

http://www.youtube.com/watch?v=jFnuc-Kxl7I