Why were medium bombers so low in numbers in Allied air forces?

Question

The medium bombers of WWII are numerous. We could mention for the Americans the B-18, B-23, B-10 to which could be added the famous B-25 Mitchell and B-26 Marauder. In the British air force, they were numerous as well: Whitley, Hampden, Wellington, strongly supplemented by the Blenheim, and one could even had the Mosquito and Beaufighter.

Given that both these air forces had or were to develop heavy bombers designs, it is not surprising that the Germans, Soviets or Japanese aligned even more medium bombers types. Some American bomber also had the possibility to have fighter-bomber characteristics, such as the B-25 which was armed with guns in order to shoot at its target before bombing it (a tactic used in the Pacific against Japanese ships).

I was not able to find extensive data, but a point came to attention when I regroup some data:

• In June 1944 in England, Anglo-American aligned 4190 fighters, 3340 heavy bombers and "only" 930 medium bombers.
• In 1943 during the Anzio battle, the raid engaged 180 fighters P-38 and P-47 and 250 heavy bombers B-17 and B-24 and apparently a small number of medium bombers because they are not mentioned

The general production of medium bombers vs heavy bombers show figures that allows to extrapolate these specific examples to the entire war:

• 18480 B-24
• 12731 B-17
• 9984 B-25
• 5288 B-26

Conclusion of prior research: There was a significantly lower number of medium bombers compared to heavy bombers and fighter-bombers in the allied air forces (British and American)

Now the question is why? I can suspect some reasons but did not find clear sources about them:

• Industrial difficulty to produce medium bombers: was it about the engine? I don't think the structure of a B-25 was more complex than a B-24's one
• Doctrine: since heavy bombers had the role of strategic bombings and also performed tactical ones (Anzio, Normandy...) their would be a stronger need for heavy bombers. But, considering B-17 and B-24 were less efficient than medium bombers in tactical bombing, I suspect they were employed there to fulfill the need of important bomb tonnage rather than for doctrinal reasons
• Vulnerability: Again a B-25 does not look more vulnerable than a B-17, and I suspect it even needs less escort because it is more maneuverable. But since B-25 and B-26 are slower (430 km/h versus 500 km/h) than a B-17, this might be an explanation?

Answer 1

The US produced 16,028 medium bombers (e.g., B-25, RB-25, PBJ, B-26, RB-26, TB-26, JM, B-34, RB-34, PV, B-23, RB-37) plus 13,651 what were termed “Light Bombers” of the twin engine variety (e.g., A-20, A-26, A-28, RA-29, A-30, RA-30, A-22) from July 1940 to August 1945.

At the same time 3760 “long range bombers” (B-29) and 31,964 “medium range bombers” (e.g., B-17 B-24, RB-24, LB-30 B-32, TB-32, and PB4Y-1 and PB4Y-2) were delivered.

So as far as bombers, generically, were concerned, roughly 54.6 percent of production went towards the long range and medium range heavy bomber types and 45.4 percent to the medium and light types.

As the USAAF was heavily invested, yea, enamored even, in the strategic bombing theory, the difference is not particularly surprising, however the dearth is not as dire as one might think when looking at individual actions. See Official Munitions Production of the United States 1940-1945, not to mention that a not inconsiderable number were lend lease and the numbers do not include the USAAF and USN two engine and USN four engine flying boats (though those numbers can be drawn from the same source) as their primary mission was patrol, not bombing - not say that those types refrained from dropping bombs or depth bombs on suitable targets.

Referenced document can tell about the number of engines produced for each type.

4/21/24 - Somewhat later; now that I thought about the subject a bit, I knew I had this somewhere and then had to find a place at which to point, see page 36.

In British production, the startling figures are that production of medium type bombers actually exceeded that of heavy bombers – 14,705 medium types (Albemarle, Buckingham, Hampden, Hereford, Wellington, and Whitley) versus heavy bomber production – 14,306 (Halifax, Lancaster, Lincoln, Manchester, Stirling, and Warwick) – 50.7% to 49.3%, without even counting those of these types specifically earmarked for maritime reconnaissance, transport or those noted as torpedo bombers, and even Mosquitos specifically noted as fighters rather than bombers, nor counting the 5071 British "light bombers" of the twin engine variety, Blenheims and Mosquitos, not included in the medium count, above, which makes the medium to heavy disparity even greater towards the medium side.

The fact of the matter, then, is that heavy bomber production amongst the western allies, was not terribly greater, if at all greater, than the medium/light types in total; there was certainly not some great gapping disparity. Care should be taken when looking at the order of battle for a particular operation or campaign and trying to draw a conclusion regarding actual production.

Answer 2

I believe here are two factors that made heavy bombers more attractive:

1. They had a much longer range. Bomb attacks would generally launch from England and have to cross the channel in order to hit their targets. More launch options opened up as the war progressed, but the targets were all a considerable distance.
2. Bomb load. Each time a target was attacked, an analysis of the bomb tonnage required to accomplish the goal was made. Heavy bombers could carry more tonnage, and more gunners, reducing the number of aircraft involved ( and at risk ). Lowering the number of aircraft involved reduced the total crew required as each bomber, heavy or medium, required a similar number of people on board.

The medium bombers did require fewer crew, but to increase the number of gunners, and therefore protection, would have made them more equal, but would have reduced the bomb load carried. Consider also, on the protection front, that, especially early in that phase of WWII, there were no/few fighters that could escort the bombers, especially as the raids penetrated further into enemy territory.

See Crew, Combat Range and Bomb load in
References:
Medium Bombers:
https://en.wikipedia.org/wiki/North_American_B-25_Mitchell#Specifications_(B-25H)
https://en.wikipedia.org/wiki/Martin_B-26_Marauder#Specifications_(B-26G)
Heavy Bombers:
https://en.wikipedia.org/wiki/Boeing_B-17_Flying_Fortress#Specifications_(B-17G)
https://en.wikipedia.org/wiki/Consolidated_B-24_Liberator#Specifications_(B-24J)

Answer 3

It is worth noting that a Mosquito could carry a greater bomb load to Berlin that the heaviest of the non-British heavy bombers. The existence of such a light-medium-heavy bomber, night-fighter, ... did a lot to fill the gap that may otherwise have existed.