Did Roald Dahl write his first book with Walt Disney?

No, Roald Dahl did not write his first book with Walt Disney. While Dahl wrote the screenplay for the Disney film "Chitty Chitty Bang Bang", he never collaborated with Walt Disney on a book.

Dahl's first book was "The Gremlins" (1943), a children's story about mischievous creatures that live in airplanes. It was published shortly after his service as a Royal Air Force fighter pilot during World War II.