Here is an overview of the B17 Bomber in BF Portal.

Walkaround and Physical Damage testing 0:00
Pilot/Bombardier 1:38
Norden Bombsight 2:06, 5:10
Gunner Positions 2:17, 6:53, 7:05
Payload various points in the video
Flight Performance 3:15 5:10
Bomb Damage Assessment 8:26, 11:43, 12:17

Walkaround and Physical Damage
Exterior model suggests DICE modelled their ingame B17 bomber after the B17E, an earlier variant of the B17 which was phased out in late 42/early 43, just before the American bombing offensive of Europe operations really started going.
The roundel on the side is an Army Air Corp (USAAC) insignia. USAAC became the USAAF (US Army Air Forces) in 1941.
The battles of El Alamein took place in 1942 between the British and German forces, so an USAAC B17E does not fit in this time frame.
Interior compartments are visible from outside.
Bullet damage decals do not show up on the surface if aircraft is damaged.
From the ground, you can only climb on top of aircraft from the horizontal stabilizers and elevators.

Pilot/Bombardier
Freelook in the cockpit is very slow. You can see all the way to the back from the cockpit.
Bombardier viewing is extremely sensitive. I could not find a way to tweak the sensitivity of these views in the Settings. I think if the sensitivity of freelook and bombardier positions were switched, it would make more sense.
The Bombardier position (accessed from Pilot position) can be accessed by scrolling down on mouse wheel (I do not know the console equivalents). The pilot will then assume the bombardier position and control the B17 in level flight via the bombsight. There is actual historical basis for this. (Historically, on the bombing run, the pilot would hand over control of the aircraft to his bombardier who would then be able to control the heading of the aircraft with the Norden bombsight.)
No power input is necessary (but you can still throttle up/down as you desire to get out of cruise speed).
When in the bombardier position, the aiming is incredibly sensitive, I could not reliably pinpoint a spot on the ground nor was it possible to accurately make out any targets on the ground with the fog the map has.
As such, for gameplay, I have to recommend using this B17 as a dive bomber, even though it was historically used as a level bomber...a real disappointment considering how they went through the trouble of adding the the wonderful feature of autolevel flight from the bombardier position which allows you to bomb from space. Previously in BF1/BFV, the pilot controlled the bombsight with his flight controls and the bombsight automatically provided a live feedback of where his bomb would land. The B17 bomber does it reversed which is more realistic.

Norden Bombsight
from the bombardier position, aim at a position on the ground and the B17 will fly towards it. Hold the fire button and the bombs will automatically release at the correct moment. The higher you are, the farther the bombs travel, the farther you will need to presight ahead. If you aim at a point already past where the bombs will land, the bombs will automatically release when you release the bombs.
When released, the bombsight will track the target until the aircraft is directly over it, from which it cannot slew back further. Use bottom gunner position to observe impact of bombs.

Gunner positions
The B17 Bomber has five gunner positions (top, bottom, tail, left, right) unlike the original BF1942 where there was only the dorsal (top) and ventral (bottom) gunner positions.
In the side gunner positions, if you look at the extremes you can actually see that the interior of this aircraft is modelled quite nicely. I suspect this aircraft was originally intended for BFV but after they ceased development, the asset was carried over and found a new life in BF Portal.
Gunners can zoom in their sights.
F3 Bottom Gunner is incorrectly labelled as Rear Gunner

Payload
There is only one payload: the 8x500lb bombs and they cannot be individually dropped.
One press releases all 8 bombs.
The bombs are incredibly weak. A 500lb bomb has the destructive power of a grenade I suspect or something very small. Do not be fooled by the big explosions. Did not notice much indirect damage, if any.
When tested on an AI server I setup, I had a difficult time getting any kills with those bombs.
When the bombs were tested on the buildings, I found they had no destructive effect whatsoever yet the engineer's EXPPACK can blow holes in buildings with no trouble.
Again, these 8x500 High Explosive bombs cannot destroy buildings, at all.

Flight Performance
Do not be deceived by the appearance, this B17 bomber flies like a Beaufighter from BFV. It is incredibly agile and can recover from steep dive angles much like a Stuka.
In the game controls settings, the Pitch Up is actually Pitch Down and vice versa for the opposite control.