BE90 FLT CREW IMMEDIATELY RETURNS TO FIELD AFTER FUEL CAP DEPARTS AIRCRAFT. SUBSEQUENT FUELING OF AIRCRAFT REVEALS NEARLY 40 GALLONS OF FUEL HAD BEEN SUCTIONED OUT.

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IMMEDIATELY AFTER ROTATION ON RWY XXR (RIGHT AFTER OUR WHEELS BROKE CONTACT WITH THE RWY) OUR R WING FUEL CAP POPPED OFF AND GAVE OFF A HEALTHY SHOWER OF FUEL. RULE #1 IS TO FLY THE AIRPLANE; AND THAT'S WHAT WE DID UNTIL WE CLRED 500 FT OF ALT. RIGHT ABOUT THIS TIME AS WE WERE REACHING FOR THE PUSH-TO-TALK SWITCH TO ADVISE TWR OF OUR SITUATION; THEY CAME ON FREQ CALLING WHAT 'LOOKED LIKE' OUR R ENG ON FIRE. WE ADVISED THEM THAT IT WAS NOTHING QUITE AS DIRE; BUT WE DID NEED TO LAND IMMEDIATELY. TWR ASKED US WHICH RWY WE WERE REQUESTING AND BASED ON THE AMOUNT OF FUEL THAT WAS BEING SUCKED OUT; I ASKED FOR AN IMMEDIATE BACK TO RWY YYL; EVEN THOUGH IT WOULD TAKE SOME MANEUVERING. TWR GAVE THE CLRNC AND AFTER A MODIFIED S-TURN WITH STEEP BANKS TO GET ALIGNED; WE LANDED SUCCESSFULLY AND TAXIED IN. AFTER TAXIING IN AND GETTING REFUELED; WE WERE AMAZED TO FIND THAT ALMOST 40 GALLONS OF FUEL HAD BEEN SUCKED OUT IN THAT SHORT AMOUNT OF TIME; SO ONE CAN ONLY IMAGINE HOW MUCH WOULD HAVE BEEN DUMPED IF I HAD CHOSEN THE FULL XWIND/DOWNWIND/BASE/FINAL APCH BACK TO THE TKOF RWY XXR!