The longest two way combat flight of a B29 was approximately 3,460 miles per Boeing Aircraft. However, this was done with a much reduced bombload, limited defensive arms, and a stripped down aircraft.
The "theoretical" range was 3,250 miles but in reality, you need to reduce this by about 15 to 20% for headwinds, lag time for formation, storm conditions, etc...and other frictions in flight. Operationally, most planned on a 2600 mile flight with some type of reserve fuel on top of that. Which usually reduced the "real range" down to "just" 2000 miles.
So, to answer your question, yes - based on the max combat mission a B29 flew, it could, with a reduced bomb load, bomb Europe from the US. With limited defensive armament, with no fighter escort, and with a very unlikely chance of many coming home. And, I suspect, limited impact.
And, just my POV, I view the B29 as the "best" WW2 strategic bomber built and used in any number.
As far as the B-36, never actually looked at that particular aircraft in this type of detail.