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?