America isn't exactly known for Heavy Tanks (unlike Germany) in WW2, however at the very end of the war they had a few projects, such as the T28, T29, T30, and T34. What would've happened if they were fielded earlier (say, D-Day, 1944), and produced in large numbers? Would they have been a detriment to the war effort? Would the war have ended sooner? I find these tank projects truly fascinating, and would love to see some interpretations as to how they might've affected Europe in WW2.
(thanks in advance!)
Counterfactuals like these are hard to answer definitively. The tanks you listed were not developed in a vacuum, but to address specific needs. Development began as a result of facing the German King Tiger tank that proved more challenging to tackle than previously known Tiger and Panther tanks. It would not be possible to field any of them for D-Day since the Americans were not aware of King Tigers before that.
But what if they were? Let's say the Americans heard rumours of some new German tanks earlier in the war and had some superheavies of their own in the back pocket. When would development have had to start in order to have these tanks see action?
Here is where we can take the Pershing as an example. A very small number of T26E3 tanks (before they were standardized) took part in the fighting in Europe in the course of Operation Zebra during the spring of 1945. A decision to build a Pershing-like tank (90 mm gun, armour proof against the Tiger) was made on April 24th 1943, more than a year before anyone knew about new German heavies. The future Medium Tank T26 would be based on the Medium tank T23 that was already in development, itself an offshoot of the Medium Tank T20 program that began in May of 1942. As you can see, it took almost two years of work to bring the Pershing from an idea to even limited service despite a lot of overlap with the existing T23 program.
With the heavy tanks you mentioned, the process took even longer. These tanks were more than half again as heavy as the Pershing and thus needed a lot more work. Prototypes of the tanks were finished years after the war: the T28 and T32 in 1946, the T29 in 1947, the T30 and T34 in 1948. Of course, development slowed down drastically after the end of the war, but also since these were just prototypes there would have been a lot of work required to make the tanks production ready even with wartime pressure.
Let's say the Americans make a very lucky guess and have some degree of premonition that allows them to concentrate development efforts on one heavy tank and begin production in just 2-3 years. In order to have heavy tanks ready for D-Day, development would have to start in the summer of 1941-42.
Let's take a look at the state of heavy tank development in the summer of 1941. Work on the Heavy Tank T1 family of vehicle was underway, with prototypes undergoing testing by the end of the year. If the alarm sounded then, the American heavy tank going into action on D-Day would look a lot like a modernized T1 (or Heavy Tank M6) rather than any of the post-war vehicles. We already kind of know what such a tank would look like. The Heavy Tank M6A2E1 was ordered as a stopgap solution in the summer of 1944 and was also late to the war. The problem was that increasing the tank's armour and armament to make it a competitive heavy tank for 1944 resulted in a clumsy 77 ton tank. Given that the Americans were fighting at the end of a trans-Atlantic logistics chain, it is unlikely that a large amount of these tanks would be concentrated in Europe for the invasion.
If we bump down development to the summer of 1942, we're looking at something a little bit more reasonable. The Assault Tank T14 was in development at the time, a 42-odd ton vehicle based on the components of the Sherman tank. The turret ring diameter of this vehicle potentially allowed for the use of a 90 mm gun (but not a "King Tiger killer" like the 105 mm or anything bigger). Unfortunately, this vehicle proved to be very mechanically troublesome even at its original weight and this kind of upgrade would have likely proven untenable. Work on this tank continued until the fall of 1943 when it was all but dropped in favour of more reliable and lighter Medium Tank T26 that loomed on the horizon.
Let's tweak the loom of fate even further. Let's say the Americans decided to go straight from the Medium Tank M3 to the T20, bringing up development of the Pershing up a year. Rather than being standardized in the spring of 1945, it would be standardized in the spring of 1944, shortly before D-Day. This lets us compared it to a number of other American vehicles put into service around that time.
Production of the Medium Tanks M4A1(76)W actually began a little earlier than that, in the winter of 1944. Even though shipments of these tanks began in April of 1944, prior to the D-Day invasion, they were not used in the initial landings. It was not believed that such a tank was necessary and that 75 mm gun was considered sufficient. The advantages offered by the 76 mm gun would be offset by a buggy new design and complicated logistics. The new tank was not requested until July of 1944. Similarly, production of the M4A2(76)W began in May of 1944 and these tanks only saw action with the Red Army in January of 1945. The Soviets were similarly reluctant to use the new tank right away, also because of logistics concerns as supplies of foreign 76 mm ammunition were sparse.
Another example is the GMC T71 (later M36). These tank destroyers were not even built anew, but converted from existing M10s. Conversions began in April of 1944, but the first M36es didn't show up in Europe until October of 1944. The M4A3E2 tanks converted in the spring-summer of 1944 also didn't make it to mainland Europe until the fall. Therefore, even if we give the Americans the foresight to standardize the Pershing a whole year sooner, it is more than likely that it would go through the same slow adoption. Commanders would be both unwilling to commit a raw untested tank to battle before a logistics chain was fully built out and likely to believe that using such a tank would not be necessary in the first place. These proto-Pershings would languish in depots and miss D-Day just as the M4A1(76)W did. Its heavy successors would then not finish development until 1-2 years afterwards, still missing the war.
In summary, the Heavy Tanks T28, T29, T30, T32, and T34 were the results of a specific stimulus and based on years of heavy tank development. Even if this stimulus came years sooner, the odds of it resulting in a heavy tank that would have hit the beaches in June of 1944 are vanishingly small. Even supernatural premonition that would fast-forward development of the Pershing by an entire year this would not aid in the development of its heavy successors to the degree that they would be able to see combat in WW2 at all, let alone at D-Day.
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