What stats might such a vehicle have? Were any of the T-20 series leading up to the Pershing good enough to actually risk putting into production? What's the earliest date that US doctrine might have been altered due to combat evidence? If a different doctrinal decision was made in early 1943, what was ready to go?
When might the latest date be? Would need to have the design finalized in time for re-tooling production lines and producing/stockpiling enough for use by D-Day. Tidworth Downs in early 1944 went with the Sherman. Would the Pershing fit into existing transport ships without displacing too much other capacity?
Is there anything about the Pershing that would have made it slower to produce than the Sherman? How much steel does it require, in terms of multiples of Shermans? Would that create a shortage? More gasoline per mile needed to operate?
Were 17pdr guns produced in the US or were they fitted in Britain?
https://en.wikipedia.org/wiki/M26_Pershing#Development
" The T20 tank reached a mock-up stage in May 1942, and was intended as an improved medium tank to follow the M4.
[9]" --quoting Hunnicutt 1996
"The
T25 and
T26 lines of tanks came into being in the midst of a heated internal debate within the U.S. Army in the mid-1943 to early 1944 over the need for tanks with greater firepower and armor."
This paper summarizes about the issues:
The Inferiority of American Tanks in World War II - George C ...
http://marshallfoundation.org/library/wp-content/uploads/sites/16/2016/04/fox_opt.pdf
He points out that no Tank Destroyer force has existed since 1946 -- a failed doctrine which Gen Devers predicted.
Also about Gen Barnes championing development of a T20 series. Both around Jan 1943.
He notes a 6 June 1945 report showing 60-90% of Shermans destroyed by burning. Which brings up the question of how much combat the wet-stowage vehicles actually saw. How many rounds fired at them vs dry stowage tanks?
Normandy attrition rates were double the expected (Italian campaign) rate. Of course, the strategic blunder of not noticing the Bocage is a whole nuther issue... Was deGaulle a complete idiot - did he fail to point it out?
Fischer Body told Barnes they were capable of T20 pilot production in Sept 1942 - but the M10 TD had priority. Not allowed to use a mere 55 tons of steel for a pair... not fixed until October.
And in Feb 1943 a Lt Col in charge of Tank Destroyers killed Barnes attempt to mount a 90mm turret the M10 Wolverine using all-stock, tested parts, saying the 3" was adequate... so the potential to show the usefulness of the 90mm on a T20 series was lost.
In Mar 1944 Barnes gave up; drafted a letter absolving Ordnance of responsibility for not having adequte tanks by late 1944. It took personal action by Marshall, in April, to finally get the authorization ball started by AGF, who suddenly ordered 6000 - but nevertheless specified 75 & 75mm guns !!!
More in the paper from the History Log T20 Series.