I look at that, and then I look at the M7 Priest:T40 3" GMC from 1941, spiked by McNair
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Original design for the M3 came this way, over the planned M2A1, that with the Fall of France, was decided to be totally unsuitable with the Germans having a 75mm armed tank
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Now with better examples of British Armor, the M3 would likely be a very interim tank, hundreds, not thousands made by Chrysler before switching over to an earlier M4 Sherman
It's worth crap as a tank, but as an SPG...
The article also notes attempts by the British to get their own designs built under contract. Now, sure, that failed OTL, but ITTL, I think the Valiant may stand a chance, given that it's, you know, the best tank in the world in 1940/1.
That not going to help an awful lot. Take a look at the plans:That's the handsome beast I was thinking of! Up-armour the superstructure and put the gun in a proper casement mount like the OTL M3 and I think you've got a fine 'rush' design in 1940/early 41 while the 'proper' tank is in development. Keep the T40 as-is to make a SPG on a common platform and the Americans are off to the races!
Just please, oh please have someone insist on having a transfer case on the radial engine so the turret basket doesn't have to be so high (I know, I know, it's a personal peeve of mine I keep bringing up but it's the one thing I genuinely don't like about the Sherman)!
You can probably only drop the height of the hull a couple of inches due to the height of the radial engine. Now if you could get the US army looking at V engines in 1940, it's possible the M3 and M4 could be lower and longer, due to the damn things not being so tall.