National Aviation Capacity Expansion Act of 2003

National Aviation Capacity Expansion Act of 2003 - Directs the Administrator of the Federal Aviation Administration (FAA) to implement the redesign and reconstruction of Chicago O'Hare International Airport in Cook and Dupage Counties, Illinois, in accordance with a specified runway design plan, and the development of a south suburban airport in the Chicago metropolitan region, by facilitating their funding, construction, and implementation and all agency reviews. Requires the State of Illinois to prepare an implementation plan under the Clean Air Act for regulating emissions associated with activity at commercial service airports.

Makes the Administrator's approval of an airport layout plan submitted by Chicago that includes the runway redesign plan conditional on the meeting of specified requirements.

Directs the Administrator to give priority consideration to letter of intent applications for the construction of the south suburban airport and for the extension of the main runway at the Gary/Chicago Airport.

Requires the withholding of all airport grant funds for O'Hare (other than grants involving national security and safety) until January 1, 2026, unless specified conditions with respect to operations at Merrill C. Meigs Field have been met.

Directs the Administrator, in providing funds to implement the runway redesign plans, to receive adequate specified assurances from the City of Chicago regarding competitive access to O'Hare by all air carriers.