Curious about this, I did a little research. Although the play was produced over seventy-five times during its first year of production (1970), the play never made it to film. I did find multiple references speaking to Hal B. Willis and a movie of the play, but all I could find about it was an article in the New York Times (July, 29, 1970) stating that Willis had acquired the rights to produce a film. The right to produce does not actually mean that a film was produced, only that Willis had the rights to do so.
One site even stated that Robert Edwin Lee and Jerome Lawrence, the authors of the play, even wrote a screenplay for the filmatic adaptation.
All of this said, I did not find a single reference to a film actually being produced. I have linked the New York Times article below, but it requires a fee to access (archived material).