When the aircraft landed and came to a full stop, it initially had a small fire on its wing that could have been putout. But because fire trucks could not get to the aircraft because the corn field was muddy and not accessible by vehicle as a result of rain fall the night before the aircraft continued to burn and eventually was consumed by fire.  

The weekend before this flight the WWII vintage Boeing B-17G had undergone maintenance for a fuel leak, the leak was repaired and a final inspection the morning of the flight there was no indication of a fuel leak. A post accident examination of the aircraft showed that the C-channel installed as part of the number 1 main fuel tank repair earlier in the week was partially separated and a longitudinal fatigue crack along the weld seam. 

The NTSB indicated the fatigue nature of the crack was consistent with a progressive failure along the fuel tank seam that existed before the accident flight and was separate from the damage sustained in the emergency landing and post-landing fire. The repair earlier in the week attempted to seal the leak but did not address the existing crack itself. 

In fact, the length of the crack observed at the time of the repair was about one-half the length of the crack noted during the post-accident examination, suggesting that the crack progressed rapidly during the course of the accident flight. 

Because the repaired fuel tank was positioned within the open wing structure, a fuel leak of significant volume would have readily vaporized, producing a flammable fuel vapor/air mixture. Although the exact ignition source could not be determined due to the fire damage, it is likely that the fuel vapor and liquid fuel encountered hot surfaces from nearby engine components, which initiated the in-flight fire. 

The National Transportation Safety Board determines the probable cause(s) of this accident to be an inadequate repair of the fuel tank that allowed the fuel leak to continue, ultimately resulting in an in-flight fire.