Bruce,
Little of this helps with your objective to connect FDR directly to the project.
The "Peacemaker" according to Joe Baugher
http://www.joebaugher.com/usaf_bombers/b36_1.html
The attached document may be even less helpful to your goal but you may as well stash them away with your B-36 files. This is a memo/letter dated 28 Sept 1945 from Norris Bradbury (Los Alamos) to Gen. Groves (MED) related to a conference held to discuss future postwar nuclear capable bomber aircraft selection.
In attendance are representative from Los Alamos, Air Technical Service Command (Engineering, Wright Field), and "Kingman" representatives Col Heflin base commander Wendover Field, Utah (where weapons development and drop testing occurred) with Capt Semple, Lead Bombardier, Flight Test Section, 216th AAF BU (Sp) Wendover, Field.
The B-36 receives honorable mention here with aircraft statistic as the only heavy bomber under consideration, nearing implementation. I suspect on page 2, item d. may be referring to what in time would become the XB-52 but that's just a WAG by me.
As suggested earlier this does not coincide with your FDR connection but does offer a window into military aircraft planning at least as it related to delivering nuclear weapons globally at this point in time during late 1945 merely weeks after the Japanese surrender.
Sorry about the individual pages, I tried loading a small PDF at 340 KB but it failed to load as too large. Also, if you want these docs please save them soon. I seem to always be running up on my limit of allowable attachments volume. These may need to be removed to allow for others in different threads. Still becoming accustomed to the limits of this forum.
Edit: Bruce, have you attempted contact this topic via request here?
https://fdrlibrary.org/contact-us