The design was discussed as early as 1940 but the first firm mention appears to be 1944 when work was started on the drawings. Dates can be a pain to define exactly. I understand the original thoughts were for more Moguls but Major Frank Pope, who took over from Malcolm Spier suggested the tank version. It is interesting that the WTs have some features more akin to the Fairburn 2-6-4 tanks like the cutaway front end and the open steps. But for the fact it was desirable to have commonality with the W class boilers, I have often wondered whether taper boilers were considered?