According to Wikipedia’s entry on Zero Hour! the Zuckers bought the rights to the film so they could use the dialog verbatim.
Airplane vs. Zero Hour (widescreen) (via flipflomas)
Proves the fact that the classic film comedy Airplane was a shot by shot send-up of the serious 1957 film, Zero Hour.
There are many cases like this in film. For instance, the 1989 movie Heathers has many shots and plot points with the 1976 film Massacre at Central High.
Just further proof that few ideas are truly original, especially in Hollywood.