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This photo shows a B-29 filled to near capacity. By counting the 500lb bombs, we arrived at a total of 16,000lb.

enter image description here

Image source - B29 Wikipedia page

This looks mostly empty. It looks as if you could fit 2-3 times as many bombs.

Why is the bomb bay so oversized?

(I tried finding a similar angle of a loaded B-52, but couldn't.)

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    $\begingroup$ It wishes it were a Lancaster :-) $\endgroup$
    – Russell McMahon
    Commented Feb 20 at 3:04
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    $\begingroup$ You're not counting the space between the open bomb bay doors as part of the bomb bay volume, are you? $\endgroup$
    – kwc
    Commented Feb 20 at 3:56
  • $\begingroup$ @kwc of course I'm not $\endgroup$
    – Abdullah is not an Amalekite
    Commented Feb 20 at 5:58
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    $\begingroup$ Lemme guess, been watching Masters of the air? So good. $\endgroup$
    – dustbuster
    Commented Feb 20 at 16:14
  • $\begingroup$ Bigger doors allow for dropping bulky items, eg food and medical supplies, if the need arose to repurpose B-52s in desperate times. $\endgroup$
    – Bohemian
    Commented Feb 22 at 1:22

4 Answers 4

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In addition to ROIMaison's excellent, referenced answer, here's a simple, practical, common sense reason:

While there is a lot of empty space around the bombs themselves, the plane can only carry a maximum amount of bomb weight. If all that empty space were filled with bombs, racks, and all the other mechanisms to load and drop them, the B-29 would have never gotten off the ground.

If that space had been enclosed with extra sheet metal to make it precisely match the amount of space the 20,000lbs of bombs needed, it would have taken extra metal, which was in short supply in war-time, would have taken extra time to construct, and would have added to the gross empty weight of the aircraft, reducing the bomb carrying capacity.

There is simply no reason to make the bomb bay "hug" the actual space that's needed for all the bombing mechanisms. The fuselage was the size it was to accommodate everything else the plane needed to do, and leaving empty space just simply isn't an issue.

An additional up side, it probably made it much easier for maintenance crews to get into the bay to check and maintain the bomb racks. This gave them some working room instead of them having to cram themselves into a Goldilocks-like "just the right size" space.

https://www.reddit.com/r/aviation/comments/1z41zo/b29_superfortress_bomb_bay_note_the_curved/

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    $\begingroup$ In addition to the above, in designing the bomb bay they may have made less dense or more awkward shaped payloads in mind. The en.wikipedia.org/wiki/Grand_Slam_(bomb) did not properly fit in the Lancaster bomb bay, and anti personnel/incendiary weapons are probably less dense/harder to pack. $\endgroup$
    – GremlinWranger
    Commented Feb 19 at 12:32
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    $\begingroup$ Sure is nice to have somewhere to put your other foot when you're working backwards upside down. And as an almost direct scale-up of the B17, it doesn't need to be any smaller, and with the circular cross section air frame, it doesn't need inner struts either. Began design in 1938; still had to be modified in Project Silverplate to carry nukes. It was already still too small. $\endgroup$
    – Mazura
    Commented Feb 19 at 19:40
  • $\begingroup$ @Mazura pretty sure they didn't have to expand the bomb bay to fit the nukes $\endgroup$
    – Abdullah is not an Amalekite
    Commented Feb 20 at 1:21
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    $\begingroup$ Well, sort of. For the envisioned Thin Man gun-type Plutonium bomb, they needed to create a single continuous bomb bay to fit the quite long bomb. In the end, the Thin Man design was a dead end, and the Fat Man didn't need the long bomb bay. However, the Silverplate program did expand the B-29 bomb bays: See en.wikipedia.org/wiki/Silverplate $\endgroup$
    – Dohn Joe
    Commented Feb 20 at 11:58
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This cut-away image shows that there are also bombs in between the two filled columns of your image.

enter image description here

Image source - 39th bomb group veterans

If only 2 bombs can be stacked in the middle (due to the forward-aft crew connection tube also being located there), we arrive at a total of (4 + 2 + 4) * 2 *2 = 40 bombs. With 500 lbs each that amounts to 20,000 lbs, which is in alignment with the payload mentioned on Wikipedia:

Bombs:

....

20,000 lb (9,100 kg) maximum over short distances at low altitude

....

This website - Nuclear Companion confirms the 40 x 500 lbs capacity: enter image description here

This cut-away confirms the 4-2-4 arrangement: enter image description here

Source - Poster Depot

This image (highlighting done by me) shows a B-29 dropping bombs, and it shows 3 bombs adjacent to each other, providing more evidence of having 3 bombs side-by-side

enter image description here

Source - Business Insider

After some searching, I found this image, which shows bombs mounted in the middle (directly below the crew transfer tube). Even though these are not 500 lb bombs, it does confirm that bombs could be mounted in this location:

enter image description here

Image source - p47koji.com

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  • $\begingroup$ It really does look like it can fit a fourth line as well $\endgroup$
    – Abdullah is not an Amalekite
    Commented Feb 19 at 10:06
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    $\begingroup$ With a fourth line of bombs you would be over the payload limit. I saw some picture of smaller bombs, they had much larger arrays, see for example this image $\endgroup$
    – ROIMaison
    Commented Feb 19 at 10:09
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One aspect, that's maybe implied in other answers but not explicitly stated: flexibility.

If your bomb bay is large, it can accommodate many different configurations easily, i.e., you reach the limit of your payload in terms of weight before you run out of space.

So, why not trim down on unused space?

Different bomb configurations may/will result in different profiles of actual space occupied. Furthermore, you may want to leave room (literally and figuratively) for future expansion.

Thus, the bomb bay needs to be large enough for any known payload configurations, plus some spare room.

Thus, for any concrete payload configuration, the bomb bay will be much too large, i.e., there's plenty of unused room.

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    $\begingroup$ For example, fitting the first nuclear bombs in. $\endgroup$
    – ceejayoz
    Commented Feb 20 at 15:22
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    $\begingroup$ @ceejayoz That was my first idea as well. Ability to accommodate unconventional loads. $\endgroup$
    – Peter - Reinstate Monica
    Commented Feb 20 at 19:04
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    $\begingroup$ It's a bit like asking "why is my outlet rated 16 amps if all I ever do is plug in my nightlight and cell phone charger?" $\endgroup$
    – Peter - Reinstate Monica
    Commented Feb 20 at 19:05
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The Short Stirling is an example of a bomb bay designed round a specific requirement. That was to carry 500lb bombs, and 2000lb armour-piercing bombs, which have similar dimensions.

Short Brothers designed a bomb bay divided into three long, narrow compartments, separated by longitudinal girders. This gave excellent strength for the weight, but meant larger bombs simply couldn't be carried: they would not fit in the space. This contributed to the rapid obsolescence of an aircraft that was excellent in many ways. Flexibility pays off in ways that you can't predict at the design stage.

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