N718CD

The pilot reported that 30 minutes after departure at 6,000 feet mean sea level, the airplane's door opened in flight. He stated that the temperature dropped, papers went flying about the cabin, and the noise was 'overwhelming.' The passenger tried to hold the door shut, but could not secured it. The pilot diverted to a nearby airport for landing. The pilot stated that he "simply forgot to put the gear down." The airplane touched down and skidded on its belly resulting in substantial damage to the airplane's stringers, formers, and main spar carry through. The pilot reported that there was no malfunction or system failure of the airplane before the accident.