On March 8, 1995, at 1800 central standard time, a Cessna 337C, N2621S, was substantially damaged while landing at the Williams Airport, near Porter, Texas. The private pilot and his passenger were not injured. Visual meteorological conditions prevailed for the personal flight. According to a witness at the airport, the airplane made the approach with the landing gear retracted. During the flare, the pilot started to add power just before the propeller blades contacted the asphalt runway. The pilot told his mechanic that there was nothing wrong with the landing gear system, and that he recalled hearing the landing gear unsafe warning horn during the flare to land. Between one quarter to one half inch of material was ground off a structural box beam on the belly of the aircraft during the ground slide. In the enclosed pilot/operator report, the pilot stated that he put the gear down and "something flapping distracted his attention, and while looking for the unusual noise, he forgot to visually check the landing gear."