According to the pilot of the Beech C90-1, the airplane was on autopilot, level, and between cloud layers, at 17,000 feet. The pilot saw some buildups ahead, and to avoid the largest one, asked air traffic control for a "slight turn" and a climb to flight level (FL) 190, which was approved. Nothing on the weather radar or stormscope indicated any severe weather ahead. Upon entering the clouds, in a right climbing turn, the airplane "almost immediately" encountered severe turbulence, and was "thrown inverted" in a nose-down attitude. As the airplane gained airspeed, the pilot pulled the throttles to idle and pushed the propeller levers full forward. He then rolled the airplane upright and had to pull the yoke "extremely hard" to recover from the dive. The event began about 17,500 feet, and the recovery was complete at 10,500 feet. The pilot then climbed the airplane to FL 190, and completed the rest of the flight uneventfully. On the subsequent preflight inspection, the pilot found wrinkling in wing sheet metal.