ORD is for American and DCA is for Delta. As always, members of CBP Trusted Traveler Programs (Global Entry, Nexus, Sentri) also have access at these locations.
Active duty military members are now eligible at DCA only (for now), the TSA Blog explains:
We recognize active duty members of the U.S. Armed Forces pose very little risk to aviation security that is why our partnership the Department of Defense is a win-win solution. This further streamlines screening for active duty service members. At DCA, active duty service members and active drilling reservists members can present their Common Access Card (CAC) to a TSA Officer in front of the TSA Pre✓™ lane, where it will be scanned to see if they qualify for expedited screening. Eligible service members do not need to be in uniform to be considered for TSA Pre✓™ benefits. It is just that simple!
A TSA Q&A indicates that military access will expand to the other airports “when operationally ready,” presumably related to scanning CAC cards.