HISTORY OF FLIGHTOn October 29, 2016, about 1540 central daylight time, a North American TB-25N airplane, N25YR, was damaged when the left inboard landing gear door separated in flight. The airline transport rated pilot, airline transport rated co-pilot, and seven passengers were not injured. The airplane sustained substantial damage. The airplane was registered to American Airpower Heritage Fly Museum and operated by the Central Texas Wing of the Commemorative Air Force (CAF) under the provisions of 14 Code of Federal Regulations Part 91 as an airshow flight. The local flight departed from the Dallas Executive Airport (RBD), Dallas, Texas, about 1500 and landed at RBD about 1545. The pilot reported that the airplane was flying about 155 mph and 1,000 ft above ground level, when the airplane entered the traffic pattern. The landing gear was lowered on the downwind leg and when the gear was in transit the crew felt a jolt as if a bird had impacted the front of the airplane. The pilot noted that the main gear extended normally, but the nose gear was slow to indicate a down and locked position. He then felt a flight shudder from the airplane and a few seconds later the nose gear down indication was confirmed. He checked the flight controls for functionality with no abnormalities noted. He made a normal landing and parked the airplane. The crew from another airplane reported to the pilot that they observed an object depart the accident airplane as the landing gear was extended in the traffic pattern. Witnesses on the ground reported observing the same event. According to the responding Federal Aviation Administration (FAA) inspector, the left inboard landing gear door separated in flight and impacted the nacelle and then the left horizontal stabilizer and elevator. The airplane made an uneventful landing at RBD where damage was observed to the left horizontal stabilizer and elevator. The landing gear door was found in a residential neighborhood about 1 mile north of RBD. AIRCRAFT INFORMATIONA review of the maintenance logbooks revealed that a Phase B inspection, as a part of the continuous inspection program, was completed on February 12, 2016. There were no logbooks entries pertaining to the landing gear door and the operator stated they did not have any discrepancies with the associated components. According to the airplane's operating manual, the maximum gear extended speed is 170 mph. METEOROLOGICAL INFORMATIONThe automated weather station located at RBD recorded wind from 170 degrees at 7 knots, gusting to 16 knots, 10 miles visibility, clear sky, temperature 82 degrees F, dew point 57 degrees F, and altimeter setting 30.05 inches of mercury. AIRPORT INFORMATIONA review of the maintenance logbooks revealed that a Phase B inspection, as a part of the continuous inspection program, was completed on February 12, 2016. There were no logbooks entries pertaining to the landing gear door and the operator stated they did not have any discrepancies with the associated components. According to the airplane's operating manual, the maximum gear extended speed is 170 mph. ADDITIONAL INFORMATIONThe CAF Director of Maintenance reported that the landing gear door connecting rod was bent and fractured into two pieces at the safety wire hole. The gear door is equipped with two arresting cables that are intended to prevent the door from hyperextending. He also reported that the arresting cables were not installed in the correct position. The investigation could not determine how long the arresting cables has been incorrectly installed. The CAF Director of Maintenance issued an internal safety bulletin to warn the other B-25 crew of the safety issue. The bulletin noted that the inner gear door attachment rod bolt failed upon gear extension which allowed the door to fly open breaking both hinges and grounding straps. The door then struck the left horizontal stabilizer on the leading edge then passed under the horizontal and struck the elevator where it tore the fabric and bent one rib. The bulletin recommended to remove the safety wire and inner landing gear door bolts and inspect the mechanical gear door linkage for signs of stress. The door connecting rods must both push the doors open and then pull them closed. Once closed, the doors are held in position in tension by these connecting rods. Carefully inspect the shorter inner adjustment bolt for any signs of bending especially near the top attachment point. TESTS AND RESEARCHThe landing gear door connecting rod assembly was found fractured into two pieces and was sent to the NTSB Materials Laboratory, Washington, DC, for examination. The examination revealed that one end of the connecting rod was outfitted with a spherical bearing rod end and the other end was outfitted with a clevis rod end. The clevis end was bent along the shank. The bend axis was perpendicular to the clevis hinge. The spherical bearing rod end was fractured in the threaded portion of the shank in the same plane as a drill hole for safety wire. There were no features indicative of a preexisting crack. Microscopic examination of the fracture surface revealed tear lines radiating away from the safety wire drill hole on the outer bend side of the fracture. The features observed on the connecting rod assembly were consistent with an overstress failure in bending.