The pilot was on the takeoff roll at approximately 90 knots when he took his hands off the throttles and began applying back pressure for liftoff. Almost immediately the aircraft began to sink, followed by the sounds of the props striking the runway. All three landing gear were partially retracted as the airplane slid along the runway surface and came to a stop with resulting substantial damage to the belly structure. The pilot and both passengers were not injured. After the accident, the pilot discovered that he had inadvertently left the landing gear switch in the "up" position while completing other items in the cockpit checklist. During the recovery operations the airplane was then jacked and the gear was lowered normally.