National Heritage Area Act This bill requires a standard for the designation of National Heritage Areas. The bill establishes a National Heritage Area System through which the Department of the Interior may furnish technical and financial assistance to local coordinating entities to support the establishment, development, and continuity of the National Heritage Areas. Such system shall be composed of each existing National Heritage Area, National Historic District, National Heritage Corridor, National Heritage Canalway, Cultural Heritage Corridor, and National Heritage Partnership designated by Congress; and each National Heritage Area designated by Congress after enactment of this bill, unless the law designating the area exempts that area from the system by specific reference to this bill. Under the system, Interior shall review and approve management plans for heritage areas, and submit to Congress reports on the activities conducted with respect to such areas. Interior may carry out or review a study that assesses the suitability and feasibility of each proposed heritage area. Interior must review and certify whether a study conducted by others determines an area meets specified requirements, including that it has natural, historic, and cultural resources that represent distinctive aspects of the heritage of the United States. The bill declares that a heritage area may be designated only by an Act of Congress. Management plans must be developed for heritage areas designated under this bill.