In the letter to FDR during WWII written by Szilárd and signed by Einstein, a rather massive (emphasis added) nuclear bomb is theorized:

This new phenomenon would also lead to the construction of bombs, and it is conceivable — though much less certain — that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove to be too heavy for transportation by air

I suppose this was before the notion that U235 could be isolated from natural uranium bringing the critical mass to a wispy 52 kg.

Regardless, what is the notional design of such a boat-delivered bomb? Is there a critical mass of natural uranium, or some mixture of natural uranium with a moderator? Was the theory to make something like the Chicago Pile, but through a to-be-determined method make it supercritical and blow up rather than melt down?

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    $\begingroup$ I think the point of putting it on a boat is because it was not known at that time if a plane would actually be able to escape the blast if it dropped the bomb. With a boat, you can just sort of push it into a port unmanned and run away, ensuring no chance of collateral damage to your side. $\endgroup$ – probably_someone Nov 17 '17 at 6:18
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    $\begingroup$ While that was maybe also a concern, the letter said "such bombs might very well prove to be too heavy for transportation by air" $\endgroup$ – Nick T Nov 17 '17 at 6:21
  • $\begingroup$ Oops, apparently I can't read. Critical mass depends pretty strongly on the degree of enrichment of the uranium; maybe the degree of enrichment that they could achieve at that point was still pretty bad? (Even at 15% U-235, you need >600 kg). $\endgroup$ – probably_someone Nov 17 '17 at 6:27
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    $\begingroup$ This letter was written very soon after the discovery of uranium fission. Science had not figured out what the critical mass of uranium would be, and indeed there was still argument over whether U-235 or U-238 was the fissile one. My understanding is they pretty much knew a bomb could be made somehow, but an understanding of what exactly it would entail was a few years out yet. $\endgroup$ – Chris Nov 17 '17 at 6:28
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    $\begingroup$ I'm voting to close this question as off-topic because it concerns history not physics. I think it belongs on History of Science SE. $\endgroup$ – sammy gerbil Nov 17 '17 at 7:04

I think it's pretty obvious (at least in hindsight and having read Richard Rhode's excellent history ) that they simply didn't know the total mass (fissile plus compression explosives plus shell) of the final device.
Side note - the destructive force is much greater when a bomb is detonated at a calculated altitude rather than ground level. They probably hadn't talked with explosives experts yet, either.

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