The pilot stated that during the landing flare he realized that he forgot to extend the landing gear and determined it was too late to go around. The underside of the airplane’s fuselage settled onto the runway and skidded down the asphalt, resulting in substantial damage to the structure of the fuselage. The pilot reported there were no mechanical malfunctions that would have prevented normal operation of the airplane. The airplane was equipped with a landing gear position warning horn that was designed to sound when the throttle lever was pulled back for landing. The pilot reported that he did not specifically remember hearing the warning horn during the landing flare; however, he stated he might have mistaken the landing gear warning horn for the stall warning horn as he was expecting to hear the stall warning during the landing flare. The pilot reported that when he went back to the airplane after the accident and powered up the airplane’s electrical system, the landing gear warning horn operated normally.