This more or less eliminates a popular speculation that the plane was mistakenly fueled with Jet A. Two people were there at the fueling. A good FBO will sample the fuel truck every morning for water, sediment, etc. This particular truck was thoroughly checked as part of the investigation.
The NTSB found no skimping on maintenance. The plane was within its annual inspection and had received progressive inspections at 25-hour intervals. Three of the engines were more or less fresh from overhaul (0 hours in January 2019; 268 hours of operation since then). Based on a quick search, I found that gently operated big radial engines in airline service after World War II were able to go 3,000+ hours between overhauls.
The plane should have been light. Out of a total fuel capacity of 1,700 gallons for a standard B-17, only 160 had been added that morning (to whatever was held in reserve from previous flying).
The report seems to eliminate another possibility, i.e., that the No. 4 engine wouldn’t feather properly, thus creating a huge amount of drag on one wing. (see ASA 2311 and ASA 529, both of wouldn’t have occurred if a prop could have been feathered; multi-engine planes are designed with the ability to twist the prop blades until they’re more or less at a knife edge to the wind, thus minimizing drag and workload for the remaining engine(s)).
The report hints at the No. 3 engine also being feathered. That would be bad. The plane isn’t designed to fly with two engines on one side and none on the other, though it probably would still be controllable at a low power setting consistent with approach and landing, especially at the “no flap” setting that they were using (flaps are essentially for landing on a short runway because they let the plane be flown slower and descend steeply at a slow speed, but they add drag and require extra power, so the pilots were being conservative in not extending them and relying on having a long runway for rolling out from a higher airspeed).
This doesn’t resolve any of the mystery, I don’t think. The failed engine was feathered, so the multi-engine plane should have been flyable just like the book says to fly it. The fuel was good 100LL. The pilots shouldn’t have needed more than a touch of power since they wanted to descend and had no flaps out.
The most surprising part of the report: “the airplane was about 300 ft agl on a midfield right downwind leg for runway 6.” Normal pattern altitude is 1000′ above ground level (AGL). “Right downwind” means they were going in the opposite direction of Runway 6 such that they’d have to turn right and right again to land. If this 300′ AGL altitude is correct, the plane was buzzing buildings on the SE corner of the airport (diagram) and better set up to land on Runway 33.
Still just as sad and still nearly as mysterious.