Looks like the chief pilot's statement doesn't specify the repair..
Exactly. Those old B17s are a complex airplane, age not withstanding. All kinds of stuff breaks between and during flights even on a well maintained aircraft. That's just the way it is. Let's not cast aspersions on what I am sure is a great group of people when we really don't know what happened.
Based on the description of events by the chief pilot, my guess, and it is strictly a guess, would be it was an electrical problem. He mentioned an the crew noticing an acrid smell a few minutes into the flight. There are no power-boosted controls in a B17, and almost everything else is electrically driven, flaps and landing gear and such. The brakes are the only hydraulic system on board.
The way the fuel system is layed out, the size and routing of the fuel lines and how the boost pumps are used, were it a significant fuel leak I would bet they would have smelled fumes in the cabin and had a more serious, visible fire initially. I saw no mention of that.
I wonder if they still had the self-sealing rubberized fuel tanks in that aircraft? We still had them in SJ up through '87 when I left the group. It was a damned expensive and difficult proposition to replace them with metal tankage back then. Even moreso now I would think.
I suppose it could have been a small fuel leak, maybe around the carburetor or in a fuel line fitting. Or leaking oil or hydraulic fluid* ignited by engine heat. I'm trying to remember how the CO2 fire suppression system is laid out in the engine nacelle. The details are fuzzy after being away for twenty-plus years, but I recall discussions about it being more of a last ditch (and really the only) defense against an engine fire.
In any event, we will know the cause soon enough when the NTSB releases its report.
eta: *If it was hydraulic fluid, it would almost certainly have to be ignited by heat from the exhaust system, maybe down around the turbocharger. There are no hydraulic lines that I remember running forward of the firewall.
anudder edit fur speeling.