Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 40; Flight Crew Total 18250; Flight Crew Type 2500
Situational Awareness; Communication Breakdown
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Flight Crew Rejected Takeoff; Flight Crew Became Reoriented
Human Factors
We were at the end of Runway 33 at DCA waiting for takeoff. Tower instructed us to line up and wait. As I was proceeding to line up and wait; Tower issued a different clearance to turn to 280 degrees and maintain 3000 ft. after departure. The co-pilot read back the new issued clearance and for some reason thought we were cleared for takeoff. I lined up and stopped. The co-pilot asked if I was ready to go. I asked the co-pilot if we were cleared for takeoff. He responded; yes. Concentrating on positioning the aircraft to line up on the runway as we were issued a new clearance and the heavy traffic on the radio at that time; I thought I might have not heard the Tower clear us for takeoff. I proceeded takeoff roll down the runway when Tower told us to abort. I believe where the miscommunication started was when the Tower issued us a new clearance while we were taxiing onto the runway; to turn to a new heading of 280 degrees and maintain 3000 ft. but did not tell us to continue to hold. After receiving the new clearance; we assumed we were cleared for takeoff. A very serious lesson to be learned here. If you are ever in doubt or unsure of any communication between you and ATC; always verify.  I have verified many times with controllers in my flying career; but this time I did not. And because of that; a terrible outcome could have happened because of it.
BE-200 Captain reported beginning the takeoff roll at DCA without clearance from the Tower.