Before this flight, the aircraft had not been flown for a period of time. The pilot was the owner of a maintenance facility and was preparing to perform an annual inspection of the aircraft. He elected to fly it around the airport to warm up the engines before draining the crankcase. Shortly after takeoff, the left engine lost power. The pilot was unable to restart the engine. He attempted to feather the propeller, but stated that it would not stop rotating. The landing gear was not retracted, and the aircraft would not maintain altitude with the gear down. The plane descended and collided with trees. Water and trash was found in the fuel lines. The pilot stated that the left engine and propeller did not impact with an object. But one blade was feathered and one was not. No discrepancies were found with the internal parts of the propeller, both blade clamps were in the feathered position. All blade clamp bolts were undertorqued.