Airplane was departing on a required maintenance test flight following replacement of both engines. Witnesses stated that the takeoff roll and initial climb appeared normal, but that at about 500 ft AGL, the airplane entered a right bank which continued until the wings were vertical and the nose fell through. Airplane impacted in an inverted steep nose down attitude. Examination of wreckage revealed that the right engine had been secured and feathered. Subsequent investigation did not reveal any evidence of pre-impact failure or malfunction of either engine or any of the airframe systems. Engine mounting/rigging continuity could not be established due to impact damage. The pilot was a principal in the operator's organization and not one of the regular line pilots. Evidence indicated that the gear was up and that the left spoiler was deployed at impact. Emergency procedure taught in transition training is to use rudder trim as soon as possible after engine failure to preclude deployment of spoilers. Rudder trim found in neutral position.