So the A20 has a lower wing loading than the 51? Hey, I'm all ears here, honestly I'm not an aeronautical engineer (yet) and am interested in this stuff. As PJ said, it is extremely counterintuitive that a twin engine light bomber like the A20 could outturn a 51.
And also, as I said, unsurprising to me.
Karnak and Bozon also mention the powerloading. I don't want to make as if the wing loading alone is determinant. Recall the equation R = (2Ws/rho CL s(theta))- max bank (theta) figures in and max bank is dependent, in turn, on powerloading and CLMax, IIRC.
However, an empty A-20 should have good Ws and decent powerloading, given the absence of the bomb load.
One question I've always had about the Radius equation... it assumes no alt loss; a flat turn. As bank increases, your lift vector available to offset weight decreases and you end up having to increase CL and Cd(induced) as well.
What if you're willing to give up alt? Intuitively, I'd expect that your max rate of turn is actually going to be realized with the lift vector horizontal toward the center of the turn circle and the a/c dropping like a stone. Of course, while doing this might put you inside the SPitty's turn, it's also going to leave you looking skyward at him, probably. But it, especially combined with some judicious rudder, might well lead to your disappearnce under the nose of an enemy seeking a lead shot.