April 9. 2023

B-17 Flying Fortress Bomb Rack

This is a 1/4 scale replica of the #3 Inboard Bomb Rack of a B-17 Flying Fortress. This is eventually going to be part of a framed wall art display piece.

This project started out pretty simple with me just designing some shackles to go along with the AN-M64 500lb Bomb that I just finished… then it eventually snowballed into design insanity.

After making the shackle I thought it would be a pretty unique piece to make a section of a B-17 bomb rack to hang on my wall as an art piece so I started to come up with a rather simple design by just looking at some various pictures that I could find online.  It was coming out pretty well I though until…

… I was doing some more research online and I found original plans for almost the entire B-17 and this lead me down a rabbit hole of redesigning the entire project. I think I went insane and decided I need to make this thing as detailed and as exact to scale as I could. (simulated blueprint drawing)

After many many hours of sifting through hundreds of blueprint sheets on Aircorpslibrary.com and probably at least 150+ hours of design time… this is what I came up with for the overall design of the bomb rack.  This would be an inboard bomb rack and it’s as detailed and accurate as I could determine using the plans that I could find. It had a few dimensions changed to make it easier to 3D print and it also might have some mixed details from both the B-17 F & G models. (some of the plans were a little hard to decipher.)

Some close up shots to show some of the details such as all of the rivets, screw heads, and larger components like the Hooks, Release Racks, and Bomb Shackle.

I’ve even included a lot of details that most people never get to see such as all of the mechanical bomb release bar mechanisms on the rear of the racks.  In my research there are very few fully complete planes in existence and most of the pictures online are of the remaining still flying B-17s’ where details (such as show above) are not restored in the flying versions.  The planes where these details have been restored are in museums where you’ll probably never get to see the inside of the planes anyway.

Here are some of the details on the rear of the rack.  Besides (the black) A-1 Bomb Release Receptacles that connect to the Release Racks on the front of the rack… all of the mechanisms here (as far as I can figure out) are for mechanical release of the bombs if the Release racks were the A-2 type.  I think the A-2 Release Racks were both electrical and mechanical but the A-4 Racks I went with are just electrical so the bomb bar shown here wouldn’t be required but I still included it anyway as it was on the plans. (There is a guard that covers the bomb bar but I haven’t added it yet. There are also covers over the A-1 Receptacles but I did not add them to show more detail.)

Here is what one fully loaded inboard rack with 500lb bombs would look like.

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The Build

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For the start of the actual build I began with the upper Bomb Rack Mount/Top Fuselage of the plane.  These are the printed parts of the mount and simulated cut away section of the Fuselage.

This part of the Fuselage is curved but for ease of printing I designed it to be printed flat.  I also printed a template that is the shape the curve of the Fuselage should be.

Using the printed curved template I cut out some matching cardboard pieces and glued them to another piece of flat cardboard.

I then glued another piece of cardboard to the curved sections to act as a form.

Here us can see how much the printed part needs to bend.  To make that happen I put the part back on the heated bed of the printer for a bit and then transferred it back to the cardboard form and finished heating with a heat gun until it bent to the correct shape.

It was a little tricky to keep all of the rails squared up but it worked out pretty well…. its not perfect but should workout in grand scheme of the project.

Next I started gluing on the other pieces.  The actual mounts glue to the fuselage rails with the cross brace in between them.  The little links in the middle would be for the bomb lift elevator.

The reinforcement bracing was then glued on to finish up the top mount piece of the project.

I then airbrushed on some OD green acrylic paint and added a little detail to the links with a brush.

Some heat-set inserts were also added to the rear which will attach this to a frame later on.

With the the top mount complete it was time to move onto the bottom mounting points which are part of the center frame/catwalk of the plane.

The bracing and lower mounts were first glued to the frame work.

The catwalk (lower piece) was printed in two piece (not shown) and glue together before painting.  The unpainted portions are where it will be glued to the frame work.

Here it is after gluing on the catwalk.

I could have done better on the finish of the frame work but it really won’t be noticeable once its mounted to the wall so I didn’t get to crazy.

Detail of the catwalk.  (The catwalk is split down the middle here…. it would run between the two inboard bomb racks so crew could get from the front of the plane to the rear.)

Next it was onto the main bomb rails. These were printed in several piece because of the size of my printer bed is only 10″ X 8″ and the overall rails are 20″ long… as it was I still had to print them diagonally on the bed.

A joiner piece helps join them together in the middle.

Once joined together the back pieces are then glued on.  Here you can see the top finished rail with the joiner in the bottom rail waiting for the back to be glued on.

Once the rails were glued together it was time to glue on the actual upper and lower mounting points.

The upper mounts slide into the rail tops and can be glued in place with a cross pin for alignment.

The nuts and bolt heads were printed separate…

Then glued on to cover the alignment pin holes.

The lower mounts are also aligned with filament pins before being glued to the rails.

Here they are after gluing them to the rails.

You can see how the bolts line up. (they look a bit off but it’s just the camera angle of the picture)

This is a filler block for the 2000 lb hook.  (Only one single 2000lb bomb could be hung on each inboard rack and this required a special D-6 Shackle and Shackle Adapter which would be hung on three hooks across the rails.)

Not that anyone will ever notice but I tried to be as accurate as I could designing this so I included the nuts inside the filler block. (Did I mention that I probably spent 150+ hours designing this thing?)

The main plate assembly also required being printed in two piece because of the print bed size limitation.

These pieces printed like garbage so I used filler to fill in the top layer lines as best as I could without getting to crazy.

It was also primed with filler primer.

It kind of filled in some of the finer details I added like all the screw heads but these were never going to show up to well with FDM printing anyway.

Here are the finished pieces after the base cote of OD green paint. Next I just need to glue them together.

Again filament pins were used to keep everything aligned.

Here is the assembled rack so far.

Now it was time to work on adding some of the detail pieces…

This first set of parts all have to do with the Bomb Release Bar (very bottom) and include the Main Crank Assembly parts, 13 Bell Crank assemblies, 7-8 Bomb Bar Supports (not all shown), and a bunch of Support U Channels for the Bomb Bar Guard. (not included).

This is how the Support U channels mounted to the Bomb Bar Supports…

… and according to the plans a cover was screwed to the top.  It didn’t cover the entire bomb bar though so I’m not sure if I will be including it into my design yet.

These are some other various mounting parts and a bunch of wire guides.  I printed the wire guides on tiny rafts and left them on for now until they are painted.

This is how small they are. They didn’t print to bad for being an FDM printer but this is when I wish I had a resin printer.

These are the Type A-1 Bomb Release Receptacles which the Release Racks plug into…  (I couldn’t find a plan for these but this picture I found online was very helpful in designing them.)

These too are little rough to print on an FDM printer but they are so small it really won’t be that noticeable once finished and mounted.

…and here we have the A-4 Release Racks. I went with the newer all electric A-4 Racks over the A-2’s because I could find a lot more “Info” on them.

For reference – This Picture Link shows an A-2 Type Rack and on the back side in the upper hand corner you can see the tab where it interfaces with the Bell Crank assemblies of the Bomb Bar so they could be maniacally release.  (At least that is my take on how I see these things working.)

Then there are the shackle hook assemblies…

There are a few different versions of the hooks. The Flat Side hook is for the 2000lb bomb and the curved hook is used for every other type of bomb the B-17 could carry. I made some of the hooks an entire assembly but the ones where bombs will actually be hung I did as a two piece with the latch separate so I could remove the bombs if I wanted. (A tiny screw will secure the latch so it can move).

I didn’t do a lot of clean up on the tiny pieces I just flashed them with a lighter before I stuck them to some cardboard with double sided tape, after that I air brushed some paint on all of them.

Next stating on the back side I started gluing on all of the detail pieces beginning with the Receptacles and Bell Cranks.

Here is what they look like from the front side.

After gluing on all 15 of the tiny Wire Clamps the next thing was the Bomb Bar, because of how it interfaces with each Bell Crank I had to align each Support bar on the Bomb Bar and then glue them all on at once as a single assembly.

Here you can see how each Bell Crank Arm interfaces with the grooves in the Bomb Bar… as the Bomb Bar slides up and down it will rotate the arm of the bell crank which in turn will turn activates the salvo release function of the A-2 Release Racks. There are three positions in the groove of Bomb Bar – Safe, Select, and Salvo (shown here in the middle would be the Select position.)

This is the linkage that would actually move the Bomb bar up and down when activated. (The actual linkage rod is not shown here.)

This is the front side of the bottom of the Bomb Bar showing the pointer that is attached to the main Plate and the selection plate. (I believe this would be used by the loading crew to see what position the rack is set to as you would not be able to see this from inside the plane.)

Next is was on to adding the finish to the Release Racks. They came out extremely well for how small they are but still retained a lot of details.

I got a little crazy and even painted a partial “Order Num.” on one of the plates just like the real thing.

Speaking of the “real thing” here is the 1/4 scale version compared to a genuine Rack Release.  (While there is red lettering on the real rack that detail was way to tiny to design in so I embellished a bit and made the “FRONT” text red instead.)

Just a quick test fit for now…

Using a 2-56 tap I threaded the top holes for all of the hooks that will hold actual bombs.

All of the complete hook assemblies are just glued in place using a filament pin for alignment.  (I did not bother to paint the hooks as I liked the plain dark grey of the filament.)

The actual working hooks will be mounted with these tiny 2-56 bolts.

The hooks are also glued in place but these allow the retainer to actually function and hold the shackles in place.

It was tedious work but here are all of the hooks after mounting.

At this point I went ahead and glued on the Release Racks too.  Now in my research I couldn’t find if every station would have a rack installed and they only cock the ones they needed or if they only installed the ones needed.  I would think they would only installed the ones need to minimize loss of unnecessary parts if the planes were shot down but this is the military we are talking about so they probably don’t think like that.  I do know that on the B-29 you couldn’t install racks in certain positions if others were installed or it would burn out the bomb lights so maybe the B-17 is similar… either way I decided to just go with the ones that would be used and one extra at the bottom just to add detail.

I went to install the bottom Framework to the Bomb Rack and I noticed I made a error designing the catwalk… I designed the wrong side so the cut out for the Bomb Rar was on the wrong side. It wasn’t a  big deal I just had to mirror the part and then reprint it.

This is what the full width catwalk would look like though.

Once that was fixed I used some temporary filament pins to hold it in place while I glued the top cat walk piece to the actual rack.

Those pins were then cut down, glued in place…

… and capped with a nut.

After repeating that process 3 more times it was time to glue in the braces that connect the bottom of the frame work to the very bottom of the rack.

Here is how that looks when done.

At this point the Frame Work is complete attached to the Bomb Rack.

In this image you can see how the Bomb bar fits through that cut out.

I had a feeling when designing the Top Fuselage mount the alignment to the rack would be off a bit because of the curve I put into it after printing it but this is way off so I’m not sure what happened.

 A quick and dirty fix was just to cut off a half inch of the Fuselage.

A couple a filler pieces and you would never know I screwed it up.

That looks a lot better. Now I just need to pin it and put on same cap nuts just like I did with the bottom mount. I also need to add some filler plug in the rear for new heat-set inserts.

Next it was onto the B-11 Bomb Shackles.  These were printed in two halves to make finishing easier.

Here it is glued together but before any clean up.

I even added a few of the cross pins although they will probably not even show once mounted.

The first test fit and everything fits great. The Release Levers of the Shackle align perfect with the Arms on the Release Rack and the shackle hooks actually work perfect and can lock the Shackle in place.   Now I just need to make 3-4 more and paint them.

Here it is test fit on the actual bomb… this is were the entire project started.

I hate making the same part more than once but… I managed to finish 2 more bombs for the rack.

To Be Continued…

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