AT ABOUT 1015 CDT, WHILE EN ROUTE, THE PLT RADIOED A FSS & REQUESTED THE KANSAS CITY WX. VMC CONDITIONS WERE REPORTED AT KANSAS CITY, BUT THE PLT WAS INFORMED OF AN AIRMET, WHICH PROVIDE A CAUTION OF LOW CEILINGS & REDUCE VISIBILITIES ACROSS HIS ROUTE OF FLT. THE PLT ACKNOWLEDGED THE REPORT & CONTINUED HIS FLT TOWARD KANSAS CITY. AT ABOUT 1051 CDT, GROUND WITNESSES HEARD A SNAP OR BOOM NOISE WHICH WAS FOLLOWED BY A HIGH PITCH SOUND OF THE ACFT'S ENG. THEY THEN OBSERVED THE ACFT SPIN OR ROLL OUT OF THE OVERCAST SKY AT AN ESTIMATED 1500 TO 2000 FT AGL. THEY SAID THE ACFT CONTINUED TO SPIN OR ROLL UNTIL IMPACTING THE GROUND. THE WITNESSES ALSO REPORTED LIGHT RAIN WAS FALLING. AN EXAM OF THE WRECKAGE REVEALED THAT BOTH OUTER WING PANELS HAD SEPARATED IN FLT. THEY WERE LOCATED 3116 & 2376 FT FROM THE MAIN WRECKAGE. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FOUND. THE NON-INSTRUMENT RATED PLT HAD ONLY 5 HRS OF SIMULATED INSTRUMENT TIME. THERE WAS NO EVIDENCE OF A PREFLT WX BRIEFING.