Hmmm...<<Former pilot and forensic analyst hat on>>
This is VERY strange indeed. The design of the B-17 landing gear is ancient but very robust...like the entire damn plane. The main gear consists of a single main strut with a single wheel. The main strut is angled forward at about 30degs and very slightly outboard. The main strut is supported in front by a secondary folding strut that locks in place when the main gear is fully extended. The secondary strut folds up into the wheel well during gear retraction. The main strut then folds up and forward during retraction. When extended, the gear lock. The gear are moved hydraulically and controlled by the pilots.
All pretty simple huh? Ok...why is Prof. Pedantic blathering on about this?
I believe that this mishap may be dur to human error. Please note in the video that both gear fail at essentially the same moment. In actually, the gear never went up or down at exactly the same time. In this video, both fail within a few milliseconds of each other, indicating a SYSTEMIC problem with the landing gear.
B-17 landing gear are VERY robust. Many times a B-17 landed with only one gear down and the damage was minimal. The good gear stayed down and the plane rolled for a while then ground-looped. In this case both failed at the same time.
www.ratol.fi/~tmannine/pics/bb_right_main_wheel_3.jpgI don't think the gear was fully down and locked on landing...OR the hydraulic system failed and the gear never locked down..OR possibly, someone retracted the gear AFTER landing.
It is possible that for whatever reason, the hydrulic system failed before landing and bled off during rollout. That means that either the mechs made a mistake or the pilots or another in the A/C made one.
Lots of work for the tin-benders, engine mechs and other guys coming up. Too bad too...pretty plane.
My guess: Pilot error...sad to say it usually is.