Yes he was. here is an extract from www.answers.com which is one of the best internet sources for this type of information. One of the world's greatest heavyweight boxers, William Harrison "Jack" Dempsey (1895-1983) was so popular that he drew more million-dollar gates than any prizefighter in history.
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It was Dempsey's savage style which gave to the sport its modern day speed. Prior to Dempsey, Boxing was a stiff, largely defensive competition, where people rarely threw punches. Basically it was two guys circling each other, and occasionally one guy threw a combination. Then it was another long wait before someone threw a punch. Dempsey just made the fights fun to watch, and demonstrated something that in the world of Kung Fu had been known for centuries; speed kills. Dempsey wasn't just powerful; he was lightning fast. There are some experts who believe that despite his weight he had so much power he could have held his own even against modern day behemoths. On the issue of size Dempsey stated he preferred bigger men because they were easier to hit.
Jack Dempsey brutalized Jess Willard for the championship, July 4, 1919. He lost the championship by 10 r. decision on Sept 23, 1926. Gene Tunney displaced him.