APPROXIMATELY 5 1/2 MINUTES AFTER DEPARTURE AND WHILE THE AIRCRAFT WAS CLIMBING ENROUTE, AN UNKNOWN IGNITION SOURCE TRIGGERED AN EXPLOSION OF FUEL/AIR VAPOR WHICH EXISTED WITHIN THE INTERIOR OF THE LEFT WING. WING SKIN FROM THE UPPER/LOWER WING SURFACE WAS LOST, HOWEVER, THE WING STRUCTURE REMAINED INTEGRAL AND THE PILOTS CONTINUED TO FLY THE AIRCRAFT EXECUTING A 180 DEGREE COURSE REVERSAL BACK TO MCCARRAN INTNL AIRPORT. SUBSEQUENT TO THE EXPLOSION A FIRE DEVELOPED WITHIN THE LEFT ENGINE AREA. AS THE AIRCRAFT CONTINUED BACK TOWARDS MCCARRAN THE AIRSPEED GRADUALLY DIMINISHED AND THE LANDING GEAR WERE LOWERED. SHORTLY THEREAFTER THE LEFT WING ENTERED A STALL CONDITION BROUGHT ABOUT BY A DECREASE OF AIRSPEED AS WELL AS THE DEGRADATION OF ITS LIFT DEVELOPING CAPABILITIES DUE TO THE EXPLOSION INDUCED DAMAGE. THE AIRCRAFT THEN ENTERED AN UNCONTROLLABLE LEFT ROLL SEVERAL HUNDRED FEET ABOVE GROUND FROM WHICH A SUCCESSFUL RECOVERY COULD NOT BE EXECUTED.