No, the first Bond film was Dr No. Casino Royale (the first Bond novel) was filmed as a 1954 telemovie before that, and once again as a movie spoof in 1967.
The novel is a fairly straightforward affair which would need to be strecthed to even fill 90 minutes. Apparently the plot of the novel with be in the movie's second half. As for the 'prequel' idea, I'm not sure if the producers are sticking with this.
Certainly, continuity-wise, it makes no sense if Dench is in the movie as M. Since she's already been confirmed on the cast list, the only alternative is if she's in it, but not as M.