N405DD

The pilot had the copilot close the cabin entry door, which later came open while the airplane was climbing through 500 feet above ground level after takeoff. The pilot executed a return to the departure airport where the airplane was landed without further incident. The door remained attached at the hinge and about half of the door skin was pulled away resulting in substantial damage to the airplane pressure vessel. The pilot stated that the CABIN DOOR annunciator was never illuminated. The operator reported that all the door latches were in the extended positions after the accident.