This jagged fragment, found in China, is from a right engine nacelle of an historic North American B-25 "Mitchell" bomber which took part in the Tokyo raid of April 18, 1942 under the command of Gen. J.H. Doolittle.
The aluminum panel has been attached to the wood plaque such that the interior of the aircraft part is facing outwards and the painted exterior is facing the plaque and not seen. This painted exterior has mostly flaked-off but some of the (presumably original) green exterior paint remains but is fragile.
This object is not on display at the National Air and Space Museum. It is either on loan or in storage.