A LEAR 25 FLT CREW IS CLRED TO BACK-TAXI ON RWY 33R AT BWI WITH ANOTHER ACFT ON FINAL; RESULTING IN A GAR.

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TAXIING OUT FROM THE RAMP AT FBO IN BWI; WE WERE CLRED TO BACK-TAXI ON RWY 33R. SINCE WE WERE AN AIR AMBULANCE AND THE PARALLEL TXWY LEADING TO THE RWY WAS OCCUPIED BY A KING AIR; WE DID NOT QUESTION THE CLRNC AND ENTERED RWY 33R AT TXWY K; THE FIRST AVAILABLE INTXN. UPON REACHING THE RWY; WE SWITCHED TO TWR FREQ AND MONITORED IT. ABOUT 1000 FT PRIOR TO THE DEPARTING THRESHOLD; WE NOTICED THAT LNDG TFC THAT ALLEGEDLY WAS ALREADY CLRED TO LAND WAS QUESTIONING THE TWR ABOUT OUR PRESENCE ON THE RWY. OBVIOUSLY; THE CTLR WAS UNAWARE OF OUR POS AS PROVEN BY THE FACT THAT HE ASKED THE TFC TO CONFIRM THE PRESENCE OF THE JET ON THE RWY. WHEN THE LNDG TFC CONFIRMED THAT THE RWY WAS BLOCKED; THE CTLR INSTRUCTED HIM TO GO AROUND. THE TFC PROMPTLY EXECUTED THE GAR WITHOUT EVER JEOPARDIZING THE SAFETY OF ANY PARTIES INVOLVED. WHEN WE REACHED THE END OF THE RWY AND LINED UP; WE CONTACTED THE TWR TO OBTAIN THE TKOF CLRNC; AND WE EMPHASIZED THAT WE HAD BEEN CLRED TO BACK-TAXI ON THE RWY. IN RESPONSE; THE CTLR ISSUED THE TKOF CLRNC AND ONCE AIRBORNE WE WERE TOLD TO CALL THEM. ONE ELEMENT THAT SURELY CONTRIBUTED TO THE CHAIN OF EVENTS WAS THAT THE CTLR NEVER INQUIRED ABOUT OUR POS AT ANY TIME LEADING US TO BELIEVE THAT HE WAS FULLY AWARE OF THE SIT. ESPECIALLY CONSIDERING THAT WE BACK-TAXIED ON THE RWY 3000 FT BEFORE THE LNDG TFC MADE THE CTLR AWARE OF OUR POS!