On October 17, 2001, about 0520 central daylight time, a Beech 58, N943V, registered to and operated by Apollo Aviation Company, Inc., was landed gear-up at the Gulfport-Biloxi International Airport, Gulfport, Mississippi. Visual meteorological conditions prevailed at the time and an instrument flight rules (IFR) flight plan was filed for the 14 CFR Part 135 non-scheduled, domestic, passenger flight. The airplane was substantially damaged and there were no injuries to the airline transport-rated pilot or two passengers. The flight originated about 10 minutes earlier. The pilot stated that after the initial takeoff, the cabin door "came ajar." The flight returned and landed uneventfully where he closed the cabin door. The flight again departed and the cabin door "came ajar again." The flight returned and the airplane was landed "...with the landing gear up." Postaccident examination of the airplane by an FAA inspector revealed that the landing gear warning system did not operate when the throttles were retarded with the landing gear retracted. Examination of the gear warning system components revealed that the microswitch for the left throttle/gear warning system was shorted internally and the roller arm was bent backwards. The microswitch for the right throttle/gear warning system was not making contact with the throttle cam; buckling of the throttle attachment plate was noted. Manual operation of the microswitch of the right throttle/gear warning system would trip the circuit breaker. The left microswitch was removed from the circuit and manual activation of the microswitch of the right throttle/gear warning system activated the gear warning system. Examination of the cabin door revealed that when the cabin door was closed and latched, the outside door handle was extended away from the door approximately 1.5 inches; a rusted and broken spring (P/N 35-400031-1) was noted. Operational check of the door latch mechanism was accomplished revealing no binding noted; the door locked. The upper latch was found to go over center with the door latched and the aft latch bolt "...locked properly." The two rivets that secure the pin guide assembly (P/N 96-420033-5) to the door were broken. A copy of the FAA inspector statement, statement from the Director of Maintenance of the operator, and of the illustrated parts catalog for the cabin door are attachments to this report. Review of the "Aircraft Discrepancy Record" sheets for the accident airplane that begin with an entry dated May 25, 2000, and end with an entry dated September 7, 2001, revealed no discrepancies pertaining to the cabin door. One discrepancy related to the gear warning system was noted; the entry was dated May 29, 2000. The entry indicates that the throttle warning horn circuit breaker popped during descent to land prior to having the landing gear extended. A copy of the Aircraft Discrepancy Record sheets are an attachment to this report. Airworthiness Directive (AD) 97-14-15, with an effective date of September 2, 1997, applicable to the accident airplane, indicates in the compliance section that the AD is to prevent unintentional opening of the cabin side door and the utility door from the interior of the airplane, which if not detected and corrected, could result in loss of control of the airplane. The AD, which is an attachment to this report, is a one-time inspection and was complied with on October 4, 1997. The airplane had accumulated approximately 1,281 hours at the time of the accident since compliance. The airplane was inspected last in accordance with a 100-hour inspection that was signed off on September 17, 2001; the airplane had accumulated 38 hours since the inspection at the time of the accident. Review of the inspection guide for the airplane indicates that the cabin door is inspected for security of attachment and the latching mechanism is checked for proper engagement and ease of operation. Excerpts from the inspection guide and the maintenance records are attachments to this report.