Very interesting that both mains collapsed at the exact same time. I'm
not familiar w/ the B17 design, but I'd guess that they are independant
mechanical mechanisms so I'd be suspicious of a control failure or
uncommanded or accidentally commanded retraction.

B-17 has a large electric (DC) motor with an angle drive gearbox on
the end driving the gear retraction/extension mechanism, if I recall
correctly (it's been about 20 years now) there's one in each gear
well. For both to retract as nicely as that, I would suspect an
electrical malfunction, maybe a relay or a short or some such thing -
can't remember how the electrics runs on those. They were used them in
the air tanker/spraying bidness right into the early '80's.

Jim