The pilot reported that he extended the landing gear while performing a 360º turn to lose altitude on final approach and verified that it was down and locked. Upon landing, the landing gear retracted, and the airplane skidded to a stop on the runway, which resulted in substantial damage to the fuselage. A witness waiting to depart stated that the landing gear was down and the airplane appeared to land harder than usual in the gusting conditions. The airplane bounced twice, and on the second bounce, while the airplane was airborne, the landing gear retracted into the up position. Post-accident examination of the airplane revealed that the gear lever was in the retracted position and the landing gear functioned normally. The pilot reported that there were no pre-accident mechanical malfunctions or failures with the airplane that would have precluded normal operation. He stated that on a previous flight, the landing gear lever became unlatched and during all subsequent flights, he would forcefully jerk on the lever to ensure it was latched.