Was the B-17's Bomb Bay Really a Fatal Flaw? I'm Doubting the Lancaster's Superiority

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Redred

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Nov 21, 2025
Hey everyone!

As an aircraft designer—even though my current projects are modern, my real passion and area of study is WWII aviation. That's why I often wonder if we focus too much on bomber payload capacity at the expense of survivability. The question is: Was the B-17 Flying Fortress truly limited by its internal structure compared to the Lancaster?

I saw an analysis claiming the Lancaster's massive 33-foot monolithic bomb bay made it superior, allowing it to carry 22,000lb Grand Slam bombs that the B-17 couldn't accommodate because of the central spar reinforcing its structure.

But I gotta disagree: I think the American decision to prioritize armor and heavy defense (the B-17's 13 .50-cal machine guns and structural toughness) was the more correct and pragmatic choice for deep-penetration daylight missions over Germany. Sacrificing defense and crew safety (like the Lancaster sometimes did on Grand Slam missions, stripping armor and turrets) for one single giant bomb isn't a sustainable long-term strategy. The B-17's combat box offered mutual defense that a lone Lancaster just couldn't match.

If you're curious, here's the video I found that makes the opposite argument (the one I, as a designer, am questioning):


View: https://youtu.be/24vBK2CBVM0
What do you guys think? Was the B-17 simply designed for a different kind of air war, or was the bomb bay design a huge structural mistake? Who won the battle of design philosophy?
 
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I believe the difference between the brilliant Lancaster and the B-17, the most magnificent aircraft to ever fly, is that they were designed in different "eras" for different missions. The B-17 was designed around 1935(?) as a "patrol and defend the coastline" kind of plane for a theoretical war. The Lanc' was the result of an ongoing evolution of the Manchester (How much crap can we dump on the Baddies?) with actual war experience as part of the Lanc's design.
I hope we will be hearing from the folks who actually know what they're talking about.
 
Until these people chime in:

I think you have your answer - when compared with the Lancaster, the B-17 was limited wrt. the bomb size it could carry.


I disagree with you here.
The B-17s combat box, at least against Germany proper, was not sustainable long-term strategy, as shown in 1943.

Redred said:
What do you guys think? Was the B-17 simply designed for a different kind of air war, or was the bomb bay design a huge structural mistake? Who won the battle of design philosophy?
Click to expand...

In the 1930s, both USAAC and RAF tried to make themselves equal to the other branches of military, and were proponents of the notion that bombers can wage their own war against a peer opponent.
The later notion (that of bombing the way to the victory) smacked the RAF in the face already in 1939/40, with the Americans were coming to the senses in 1943. By then, a lot has changed, with the B-17 predating that 5+ years; Lancaseter was a younger design, and more lessons learned could've been aplied. One of them was that night bombing is healthy for both for the crews and bombers.

Lastly, we should not judge the B-17 too harshly. It was one of the best bombers of ww2, and there was a lot of the targets where the 'normal' bombs will do the job. That the 'bomber will get through' mantra was a bad one was not a fault of the B-17, nor the people that made it or flew it.
 
The "B-17s combat box" might have worked if the Germans were attacking with Bf 109Fs and early Gs. (single 20mm and pair of 7.9mm mg)
They didn't and that should have been foreseen. Both the British and Americans were building fighters with much heavier armament or planning to and a prudent planner would have predicted that the Germans would do the same thing. Strap extra 20mm guns under the 109, use the Fw 190 as an interceptor, use a twin for attacking the bomber box. Expecting the enemy to make few or no changes over 1 1/2 to 2 years from initial planning to the wide scale deployment of the B-17 combat box is poor planning.
Perhaps the combat box took a while to develop but the first B-17F flew May 30th 1942 so planning had been going on somewhat longer. The "G" was a late arrival. The Nose guns on the G helped some but did not really change the fact that unescorted missions were a bad idea.
 
I tried listening to that video & gave up on it. Sounds like a AI generated video to me that seems to have the Lancaster appearing miraculously out of nowhere as some kind of wonder machine.

Sept 1936 the Air Ministry issued Spec P.13/36 for a medium bomber. Amongst the requirements were the ability to carry an 8,000lb bomb load internally or 2x18" torpedoes. It also had to be capable of being catapulted from an airfield and to dive bomb at 60 degrees (at least in part due to the torpedo dropping requirement), as well as acting as a troop transport. Out of that proposals were made for what became the Avro Type 679 Manchester (powered by the RR Vulture) and the Handley Page HP.56 (with options put forward for 2 Vultures or 4 Merlins).

Then the Air Ministry began altering the Spec.
26 Aug 1937 - cancellation of the torpedo dropping requirement
4 July 1938 - cancellation of the catapult requirement
11 August 1938 - cancellation of the dive bombing requirement
1940 saw the troop transport requirement dropped

Avro were the first choice and received an order for 200 production aircraft off the drawing board on 1 July 1937, such was the urgency of getting the new type into service. The torpedo dropping requirement is shown in drawing as being two carried in line rather than side by side thereby accounting for the long bomb bay (An interwar Mk.XII torpedo was 16ft 3in long). Only much later, when it became clear that the Manchester needed re-engined, did Chadwick admit to senior officials at the Air Ministry that his inexperience of designing large all metal aircraft of the rigidity demanded by the specification resulted, at least in part, to the aircraft coming out overweight. Since I found out about this I have often wondered if that overengineering was why the Lancaster subsequently was found to have the strength to lift those big bombs. But as the Spec was changed Avro were not permitted to redesign the Manchester to avoid delays. For example catapult trials of the Manchester took place in Sept 1942 at RAE Farnborough.

Initially it was the Vulture powered HP design that was chosen, but on 3 Sept 1937 HP were told to proceed with the 4 Merlin option instead. That caused there to be a complete redesign in the latter part of 1937 which became the HP.57 Halifax in due course. That was accompanied by an order for an initial 50 aircraft. Without the torpedo dropping requirement the bomb bay could be made shorter.

By mid-1940 Chadwick, having realised that problems with the Vulture even if resolved would still not do much to improve the Manchester, began work on a four Merlin engined "Manchester III" in mid-1940 that was later renamed the Lancaster. But as it was being designed there was little expectation that it would prove to be better than the Halifax. Its adoption was however an easier solution in the midst of a shooting war to getting an effective bomber than trying to convert the Manchester production facilities over to Halifax production.

When it comes to the big bombs, there were 3 to consider:-

12,000lb HC - an 8,000lb HC bomb with an extra section added.
12,000lb MC (Tallboy) - 835 dropped operationally on targets between 8 June 1944 & 25th April 1945
22,000lb MC (Grand Slam) - 40 dropped operationally on targets between 14 March and 19 April 1945 with the largest single operation seeing 12 dropped on 27 March on the Farge U-boat shelter. The aircraft dropping these weapons had to be further modified from the Tallboy configuration to reduce the basic aircraft weight. So they lost the bomb bay doors (fairings fitted in their place) and nose and mid-upper turrets which were the most obvious changes, but there were other internal ones.
 
EwenS said:
I tried listening to that video & gave up on it. Sounds like a AI generated video to me that seems to have the Lancaster appearing miraculously out of nowhere as some kind of wonder machine.
Click to expand...
Thank you. Seeing that thumbnail is an AI-generated image, and the bombastic title - clear signals to stay clear of the video.
 
Redred said:
Was the B-17 Flying Fortress truly limited by its internal structure compared to the Lancaster?
Click to expand...
Yes, the B-17 was limited to 6,000 pounds of HE bombs carried internally, maximum individual size 2,000 pounds. WWII bombing showed it was the heavier bombs that were more likely to cause lasting damage, the heavier bombs were also more accurate, though individual bomb designs varied. Using the 22,000 pound bomb as a criteria for the Lancaster is rather like using the Disney bomb for the B-17, a special mission rare event in the context of the war.

Most of the larger bombs dropped by Bomber Command were by Lancaster, 41 Grand slam, 828 Tallboy, 212 12,000 pound HC, 1,088 8,000 pound and 89,333 4,000 pound for Bomber Command as a whole on bombing missions.

Redred said:
I think the American decision to prioritize armor and heavy defense (the B-17's 13 .50-cal machine guns and structural toughness) was the more correct and pragmatic choice for deep-penetration daylight missions over Germany.
Click to expand...
Self defending bothers were not. The more guns the more drag, making interceptions easier, it is remarkable how many interception sorties failed to make contact at times, the more crew on board to man the guns the more casualties when the aircraft was hit. In April 1943 the commander of the 306th Bomb Group, Colonel Putnam, wrote a report, problems with aircraft identification, the speeds involved meant only around 10% of gunners who could have fired during the crucial 2 to 3 seconds of a fighter attack did in fact do so.

While the tactics and use of heavy machine guns upped the interceptor loss rate it still came down as a win for the interceptors, 8th Air Force heavy bomber versus Luftwaffe fighters was something like 3 USAAF to 2 Luftwaffe losses in 1943, 2 to 1 in early 1944, before the 30mm cannon came into deployment.

No defensive armament makes it safer for an interceptor to close before opening fire, more defensive armament the greater the number of interceptors able to line up an attack. It is a trade off. In daylight a least some armament was desirable. The tactics of tight formations at altitude upped the fuel consumption, Roger Freeman indicates a B-17 strike at 320 miles used around 1,645 US gallons of fuel, while a Lancaster at that range around 1,000 imperial gallons. The B-17 had 5,000 pounds of bombs, the Lancaster had over 12,000 pounds available for bombs. Ignoring fuel reserves at bomb release the B-17 was carrying around a thousand pounds more fuel.

The lower the average bomb load the more bombers needed to deliver the tonnage, the more chance dust and smoke will reduce accuracy and since entering ground fire zones was a percentage bet, the more bombers lost to ground fire, which accounted for about half of all 8th Air Force heavy bomber losses.

Having said all that the average B-17 load in the 8th Air Force was 5,140 pounds, the B-24 5,320 pounds, the higher flying B-17 had fewer weather related abortive sorties, but in 1944 the number of B-17 lost or written off was 2% of credit sorties, the B-24 1.8%, then comes if, like Lancasters, the B-17 was sent to the tougher targets at times.

In summary the B-17 bomb bay was a real issue, but the 8th Air force tactics, targets and opposition limited the effect in terms of average bomb loads. The smaller bomb loads allowed the extra armament. The B-24 could carry 8,000 pounds of HE bombs internally.

A Lancaster with Merlin 60 series engines and so closing the altitude performance gap with the B-17 would need to remove the front and mid upper turrets to compensate for the extra engine weight, it would also be about 25% faster in that configuration, clipping an hour off a mission to Germany, cruising at about 300 mph, a speed the escorts were more comfortable with.
 
That point about the design era is absolutely crucial, and I think it perfectly highlights the constraints the B-17 designers faced. You're right—it was essentially a pre-war design built for a theoretical mission (patrol/defense), which meant structural integrity (like that central spar) was probably non-negotiable at the time.

The Lancaster, conversely, was a direct product of battlefield necessity and the realization that the initial designs needed much bigger payloads for area bombing. It's a perfect example of design evolution in wartime.

It makes me wonder: do you think the USAAF should have been faster to adapt its heavy bomber design once the war showed them the B-17's structural limits on specialized ordnance? Or was the sheer number of B-17s produced (and their survivability) a better strategic asset than starting a new design like the Lancaster?
 
This is exactly the kind of historical context I was hoping for.

I totally agree with your analysis on the pre-war design philosophy—the "bomber will get through" mantra—and how both air forces were forced to adapt quickly once the fighting started. That difference in design age (B-17 from '35 vs. Lancaster's evolution) explains a lot of the structural rigidity/limitations.

On your counterpoint about the combat box not being sustainable: You're absolutely correct about the brutal losses in '43. But doesn't that reinforce the pragmatism of the B-17's heavy defensive focus, even if it wasn't a perfect strategy? If the USAAF was committed to daylight precision bombing, they had to double down on defense, even if it meant structural compromises on payload (the spar) and heavy crew losses. They chose to survive the mission, even if they couldn't carry the Grand Slam.

It's a fascinating choice: High survivability + limited payload (B-17) versus High payload + lower survivability/crew risk (Lancaster/Grand Slam missions). It really comes down to which type of risk was deemed acceptable at the design table.
 
You've hit on something really important: static threat assessment in design is a killer. It's an almost unforgivable flaw for planners to assume the Luftwaffe would stand still for 2+ years. The rapid introduction of things like the Fw 190 and heavy cannon packs (which you mentioned) fundamentally invalidated the "self-defending bomber" concept almost immediately.

As a designer, this is the core failure: if the USAAF had predicted the actual threat level, would they have focused less heavily on cramming in all those .50-cals and that structural spar, and instead gone for a design with more flexibility for specialized ordnance?

The B-17 was designed for the expected war, but the Lancaster was adapted for the actual war. That distinction, I think, is what made the difference in payload capacity. Great perspective!
 
thank you for this incredible background detail! That breakdown of Spec P.13/36 and the evolution from the Manchester is exactly the kind of expert insight I was hoping to see here.

I totally get your point about the video's presentation style. Sometimes videos trying to cover complex history simplify things a bit too much, which can make the narrative sound a little "miraculous."

However, the details you provided perfectly underscore the core of the debate:

You mentioned that Chadwick's inexperience might have led to overengineering and increased rigidity, potentially explaining why the Lancaster subsequently could lift those huge bombs. If the Lancaster gained its key strength almost accidentally through a design evolution (the long torpedo bay leading to the Grand Slam capacity), doesn't that make the final difference between the B-17's fixed mandate and the Lancaster's evolutionary design even more fascinating?

Great historical context—thanks for taking the time to write it all out!
 

USAAF's mantra was the daylight precision bombing, reality was that the 'precision' part was lacking.
Everyone but Japanese was focusing on survivability, but what was designed in did not always worked. Germans and British switched to the night bombing, while the USAAF introduced the LR escort fighters to make the bombers survive. Just by choosing to survive the mission was not enough for the USAAF bombers.

I would not worry too much on the B-17's inability to carry the Grand Slam...

Redred said:
It's a fascinating choice: High survivability + limited payload (B-17) versus High payload + lower survivability/crew risk (Lancaster/Grand Slam missions). It really comes down to which type of risk was deemed acceptable at the design table.
Click to expand...
... and I would not be so much hyped about the supposed survivability of the B-17s, at least not when flown against the capable defenses.
 
Tomo pauk, you're hitting on the core strategic reality here!

You are absolutely right that the B-17's internal defense structure wasn't enough on its own. The LR escort fighter—the P-51—was the true solution to the survivability problem. Without it, the "combat box" was just a target-rich formation, as we saw in '43.

But let's bring it back to the design table:

If the USAAF had acknowledged the need for long-range escort from the beginning, do you think they would have still committed so much design weight and internal space to the 13 machine guns and the spar that limited bomb size? Or would they have stripped down the defense, focused on speed/payload, and relied more on the escort, moving the design closer to the Lancaster's philosophy?

It seems the B-17's eventual success was due to the external asset (P-51) compensating for the flaw in the original design mandate. Thanks again for this great perspective on how doctrine and design clashed!
 
The firepower of the Lancaster & Halifax was upgraded towards the end of WW2 with the introduction of tail turrets with two 0.5" MG in place of the 4 0.303" MG.

RAF Bomber Command ran a successful strategic daylight bombing campaign from Aug 1944 to April 1945, alongside their continuing night bombing. 153 missions targeting oil and transportation targets in particilar that were more easily hit accurately in daylight. They didn't use the tight box formations of the USAAF so saving the issue of retraining crews and easing the fuel burn as aircraft maintained station, preferring looser "gaggle" formations.

But that switch was helped by Fighter Command being freed up from the anti-V1 campaign and the increasing availability of long range Mustang fighters. The end of Sept saw the last Mustang wing in 2nd TAF being returned to Fighter Command to join the escort force. More squadrons switched from Spitfires to Mustangs from then until April 1945.

Tallboy & Grand Slam were never Bomber Command main force weapons. They were specialist weapons dropped in limited numbers, against very specific targets by just two squadrons during WW2 who trained to do only that. 617 squadron began dropping Tallboys in June 1944, being joined by 9 squadron in Oct 1944. Post war the capability was picked up by 15 squadron by for trials against various hardened targets as 9 & 617 reverted to being main force squadrons. So it is just wrong to view that as some kind of typical Lancaster bomb load. Details of some more typical loads can be found here and were dictated by the type of target and purpose of the raid.

Bomb Loads

Lancaster Archive
www.lancaster-archive.com

Tallboy and Grand Slam casings were constructed in both Britain and the USA. USAAF trials with Tallboy on a modified B-29 were undertaken in spring 1945 which led to their participation in post war trials like Project Ruby and the equipment of a single B-29 Bomb Group with the capability, which was used to a minimal extent in Korea.

Tallboy / Grand Slam missions were more akin to the limited use of the Disney Bomb by B-17s of the 92nd BG in 1945. Some of their B-17s carried two 4,500lb Disney Bombs externally to attack hardened targets, but these were short range missions on coastal targets in Germany & the Netherlands. 158 were dropped in WW2. But hardly a typical B-17 bomb load. There is a short period newsreel of their first operation showing them being loaded and dropped.

Disney bomb - Wikipedia

en.wikipedia.org
 
Spar would've probably remained as-is, since USAAC in the 1930s didn't have the huge bombs in their inventory.
If having the powerful escort was in the doctrine, there would be probably a more modest defensive firepower installed, talk half a dozen of LMGs initially, to be later replaced with 5-6 HMGs?
 
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