Admiral Canaris
Banned
If the Allies are as successful in cracking German codes as they were OTL, then that will probably be their #1 priority above and beyond the other strategic bombing campaigns. They'll throw everything they have at it until the facility (and the people involved) ceases to exist. And not just bomber strikes, either: as much sabotage as the OSS/MI-5/resistance can mount to disrupt the supply and support network in coordinated effort to cripple as much of the project as possible.
Problem with sabotage is, a bomb factory would most likely be situated in Germany proper, where there was no resistance to speak of. SOE operations there would be severely hampered.
The B-36 wouldn't have been a good platform for a WWII-era nuke. It was slow and unmanuverable, and would have been a sitting duck for most German interceptors (particularly if Hitler is smart and orders the Me-262 tasked as an interceptor and not a tactical bomber). In addition, it was a maintenance nightmare and was as likely to be lost to mechanical failure as to enemy action.
The "Featherweight" B-36s (with unnecessary extraneous equipment such as turrets, as well as a lot of internal equipment, removed) reached well in excess of 50,000 feet. This is above the effective service ceiling of pretty much anything the Germans can reasonably throw together. They are hardly easy targets.
The B-47 would be a much better choice and within WWII Allied technological capability (it was already in the design stage as early as 1943; the earliest model was basically a B-29 fitted with jet engines). If the war drags on somehow into the late 1940's, then the far superior B-50 and B-52 are available, but German interceptor technology is likely to advance to meet the threat as well.
Anything recognisable as the B-47 will not come into service until towards the end of the '40s at the earliest; the B-36 will be much quicker to introduce, with less troubles to straighten out and being already well into development. Nonetheless, even IOTL, only a relatively few B-36s were available as late as 1948. B-52s are right out of the question; nothing in the POD allows technology to "skim" half a decade or so of development. Also, the B-47 doesn't approach the service ceiling of the B-36; it will be much more vulnerable to German interceptors.
Keep in mind also that the USA also has significant rocket expertise of its own in Robert Goddard and his liquid-fueled rocket projects. Combined with the already extensive British rocketry research and superior Allied electronics, an inertially-guided missile just might be at the bare edge of practicality. Maybe not to deliver an allied nuke itself, but to pound the living crap out of the suspected German bomb factory and save a few thousand bomber crewmen's lives...
I doubt the Allies could produce that kind of accuracy in a ballistic missile that early.