"Airplane!" is a spoof of one movie - "Zero Hour"

I had always assumed it was a spoof of generic disaster movies, but as the video clearly shows, it was based on the plot of the 1957 movie "Zero Hour," often utilizing the same dialogue.  The creators of this movie accidentally videotaped the 1957 movie when they set up their VCR to tape late-night commercials they could spoof.

Credit to GromBlog, via Bifurcated Rivets and Ravings of a Semi-sane Madwoman, all of whom posted this several days before the big blogs picked it up.