AN ACR LTT MAY HAVE TAKEN OFF WITHOUT CLRNC. IT WAS CLRED INTO POS.

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FLT WAS #1 FOR TKOF ON RWY 30 AT IAD. COMPANY ACFT HAD TAKEN OFF APPROX 45 SECONDS BEFORE WE REACHED END OF RWY. TWR CLRED US FOR TKOF (WE THOUGHT); WHILE WE WERE STILL APPROX 100-200 YDS FROM RWY END. AFTER COMPLETING OUR BEFORE TKOF CHKLIST; WE BEGAN OUR TKOF ROLL AS NORMAL. WHEN WE WERE ABOUT 500 FT OFF THE GND; TWR CALLED TO SAY WE WERE 'CLRED FOR TKOF.' AFTER MY FO AND I EXCHANGED STARTLED LOOKS; THE TWR; AFTER OBVIOUSLY SEEING US ALREADY AIRBORNE; SAID; 'OH; UH; OK; I SEE YOU'VE ALREADY TAKEN OFF;' (OR WORDS TO THAT EFFECT). THIS ALERTED BOTH MY FO AND ME TO THE FACT THAT SOMEBODY (US OR THE TWR) HAD MADE A MISTAKE. WE ARE STILL NOT SURE. I FEEL 95 PERCENT POSITIVE THAT WE WERE INDEED CLRED FOR TKOF. CONTRIBUTING TO THIS POSSIBLE ERROR IS THE FACT THAT WE WERE CLRED (INTO POS OR FOR TKOF) WHILE WE WERE STILL SOME 200 YDS FROM THE END OF THE RWY. I FEEL THAT AFTER COMPLETING OUR TKOF CHKS SO FAR FROM THE RWY; WE HAD CONDITIONED OURSELVES INTO THINKING WE HAD BEEN CLRED FOR TKOF AND NOT JUST 'INTO POS AND HOLD.' I ALSO POSSIBLY HAD CONDITIONED MYSELF SINCE I SAW THE PLANE IN FRONT OF US AS WELL CLR AND KNEW THAT NOBODY WAS ON FINAL. SUPPLEMENTAL INFO FROM ACN 248270: THE TWR CTLR CLRED US ON TO THE RWY; THEN FOR TKOF WHILE WE WERE STILL A DISTANCE AWAY FROM THE THRESHOLD. APPROX 100 FT AGL THE TWR CTLR SAID; 'ACR LTT CLRED FOR TKOF; ON CONTACT DEP.'