THE PLT WAS ISSUED RWY 5, 7,500', FOR IFR DEPARTURE WITH FAVORING 4 KT QUARTERING HEADWIND. PLT REQUESTED RWY 14, 4,498', FOR TAKEOFF WITH A 4 KT QUARTERING TAILWIND. GROUND WITNESS AND CONTROL TOWER OBSERVED ACFT USE NEARLY ALL OF THE RWY ON TAKEOFF ROLL. AFTER AN ABRUPT ROTATION, THE CONTROLLER OBSERVED ACFT YAW TO LEFT AND MAKE A LOW ALT LEFT TURN. SECONDS LATER THE ACFT RAPIDLY DESCENDED INTO TREES AND CAUGHT FIRE. THE LEFT PROPELLER WAS FOUND IN THE FEATHERED POSITION AND THE LEFT ENGINE WAS CONSUMED BY A GROUND FIRE. THERE WAS NO EVIDENCE OF ANY INTERNAL ENGINE FAILURE. THE ENGINE TIME SMOH WAS 43 HRS. WITNESS HEARD THE ACFT TAKEOFF WITH A SERIES OF LOUD BACKFIRES 25 DAYS PRIOR TO THE ACCIDENT. INJECTOR NOZZLES ON THE RIGHT ENGINE WERE LEANED TO CORRECT THE PROBLEM AFTER 6 HRS OF OPERATION ONE WEEK LATER. THE PLTS LOG FAILED TO SHOW ANY RECENT TRAINING IN SINGLE ENGINE PROCEDURES.