Dulles Airport’s IAD identifier is derived from the initial letters of its name, Dulles International Airport.
When Dulles originally opened in 1962 it was given the logical airport code DIA. It changed to IAD in 1967 to avoid confusion and accidental fight routing errors with nearby Washington National Airport (now Reagan National Airport) which uses the code DCA.
IATA has since assigned the DIA location code to Doha International Airport, Qatar.
