One of the neat things about persuing old magazines is that you get nuggets of interesting information. I'd believed that the Spitfire was R. Mitchell's last design development, and it probably was the last - to be flown. However, it appears Supermarine had started on the design of a heavy bomber (to the same specification that turned out the Short Stirling) that didn't go much beyond the mock up stage. It had been ordered as a backup in case the Stirlling ran into problems, but since Supermarine was deeply engrossed in a pesky single seat, single engine design, it didn't have the resources/time to work too hard on this.
If developed, most of the bomb load would have been carried in wing cells, the inner cell of which each could carry two 2,000lb bombs, and the fuselage bomb bay could hold three 2K lbs.
For its size, it would have been lightly armed - gun turrets fore/aft, and provision for beam guns.
Had it gone into war production and development, I think it would have ended up with dorsal turret(s), and Merlin engined variants were envisioned.