The character has seen a few variations over the years - beginning as a series of books in the 1920s, continuing on to the Rex Harrison portrayal in 1967's Academy Award winning "Doctor Dolittle", and the latest portrayal done by Eddie Murphy in 1998's "Doctor Dolittle" and the sequel in 2001.
This will, no doubt, be a great role for Downey, seeing as how versatile an actor he is, portraying a superhero, a literary legend and various dramatic roles, as well.
Stephen Gaghan wrote the screenplay and will also direct the film.