At approximately 1700 mountain time, a Beech A35 was damaged in a gear-up landing at Mondell Field. The pilot was not injured. The pilot said he had just purchased the airplane and was making a local flight. When he returned to the airport for landing, he forgot to lower the landing gear and inadvertently landed wheels up. The airplane is equipped with an audible landing gear warning system that is activated whenever power is reduced below 12 inches of manifold pressure and the landing gear is still retracted, but the pilot said he didnt know if his airplane was so equipped. In any case, he said, he didnt hear the warning horn prior to touchdown.