The building you're referring to was one of the many old hangars at DCA that were built as the airport expanded from the 40s to the 60s. I can't really put a date on it, but if you're ever at DCA, mosey on down to the Main Terminal, and there's a photo exhibit on the temporary wall shielding passengers from construction which sheds some light on the airport's history.
When the North Terminal and the Commuter Terminal were demolished in 1989(?) to make way for the beginning of development for a new terminal (which would later become National Hall, aka the B/C terminal), many airlines were left without a home, including Delta, Pan Am and USAir/USAir Express. To solve this, the hangar in question was converted over into a building called the Interim Terminal, which housed something like thriteen gates--ten for USAir/USAir Express, and three for Delta and the Delta Connection. Pan Am/Pan Am Express found themselves moved into three gates on the second Main Terminal pier (15, 16, & 17), which has since been demolished too.
When the new terminal opened in 1997, Delta and USAirways moved in, and the Interim Terminal was no longer needed. This was according to plan, and they started to convert it back to a hangar; it was never meant to be a permanent solution, only a temporary one. I believe that, in addition to serving as a hangar in the future (present?), the MWAA (Metropolitan Washington Airports Authority) holds office space in the former Delta gate area, which protrudes out of the hangar towards the end of National Hall. If you visit there now, you'll be able to see the MWAA logo displayed prominently on the side.
I'm guessing that in the future, when the gates at the new terminal become too crowded, they might knock down the hangar and replace it with a commuter/regional jet terminal to serve USAirways Express, Air Canada, American Eagle, Comair, etc. It's just a guess, but I think it makes sense.
Hope that helps!