"The Forbidden Planet" came out in 1956, and there is a particular visual effects shot in the movie that was ground breaking at the time. Projecting live action onto a painted background, and I've always thought this was the first usage of this VFX technique in a motion picture. Particularly the way it was done, which I'm not sure how it was done, but I think it involved filming the live action with a painted glass plate as the background.
Here are the shots I'm referring to.
I found an interesting blog on the matte paints done for The Forbidden Planet. With some amazing work in progress shots of those scenes. I hadn't seen these before.
That article does point out on interesting point about one of those shots, and that was a MOVING CAMERA. I think this is what I was looking for, and might be the first time a moving camera was integrated with a live action matte painting effect.