Not exactly. There were a number of different breeds used in "creating" the Boxer. One of which was a white bulldog. Later came Meta von der Passage who is credited as being the "mother" of all Boxers and she was mostly white. The earliest Boxers were mixed colours but yes had a lot of white in them. Not long after the breed was created the Munich Boxer Club started breeding out the white colour because they considered it undesirable.
On the American Boxer Club website, they have a very good short history of the breed.
ABC History of the BoxerThere are many good books - some of them very old and hard to get hold of. Perhaps your Boxer Club or the Canine Authority has a library where you can access some of these classic boxer books. Sometimes they pop up on Ebay and sometimes at dog shows. If you can find any of them, snap them up.