Ugh, I have heard so many bad things about those Bendix turrets. I'd almost rather take my chances in a ball turret. I am not aware of any retractable B-17 ball turrets either. They did have tool kits that were used to jettison the turret in the event of a wheels up landing. If the turret was dropped before landing, the B-17 could be back in the air in a matter of days. If the ball stayed on, it would break the back of the Fortress.
Below is a shot I took in Nine-Oh-Nine last year. It's kind of hard to see the support post going all the way to the top of the fuselage because of the wiring harness above the O2 bottle.
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