Seems like the DOT tentatively awarded the slots (http://www.regulations.gov/document/...024-0065-23579). Emphasis is mine.

"By this Order, the U.S. Department of Transportation (the Department) tentatively grants two slot exemptions at Ronald Reagan Washington National Airport (DCA) to each of five nonlimited incumbent or limited incumbent air carriers, to operate new nonstop roundtrip service as follows: • Alaska Airlines, Inc. (Alaska) for service to San Diego, California (SAN); • American Airlines, Inc. (American) for service to San Antonio, Texas (SAT); -2- • Delta Air Lines, Inc. (Delta) for service to Seattle, Washington (SEA); • Southwest Airlines Co. (Southwest) for service to Las Vegas, Nevada (LAS); and • United Airlines, Inc. (United) for service to San Francisco, California (SFO)."

AA plans to use an A321, daily.
DL plans to use an A321neo, daily; will compete with AS on the route.
AS plans to use a 7M8; already serves via IAD. SAN is the largest unserved market from DCA, and last served by US in 2014.
WN will compete with AA on the route.
UA will add a second flight to SFO, and of course already serves via IAD.
(Details reproducted from AirlineGeeks, any errors or omissions are theirs. )