The failure of the pilot to use the checklist resulting in a gear-up landing. Contributing factors in the accident were: 1) The inoperative landing gear warning system, 2) Inadequate preflight of the airplane by the pilot for his failure to assure that the cabin door was closed and latched before takeoff, and, 3) Distraction of the pilot due to the open door. Findings in the investigation were several discrepancies related to the cabin door.