Long time gone since the last GOT, I know and am sorry. The more as it is really difficult to write something about this big bird what is NOT found in Wikipedia.
Only thing I can do is to construct this GOT as "Duel": between this one and its predecessor, the B-29, and what became the latter's successor, the B-50.
The main, or say only, difference between B-39 and -29 are the former's Allison inline engines. And the difference in performance is impressive:
Speed: 648 km/h vs. 570 km/h
Range: 10,060 km vs. 9,100 km
Ceiling: 11,000 m vs. 9,170 m.
The B-50 is nearly equal in ceiling (11,200 m), a bit better in range (12,472 km) and, slower in speed (634 km/h) than the B-39. The B-50 runs again on radial engines. It looks like the use of inline engines of highest power was then near to a close, while the one of radials still continued for a while.
To conclude is that the B-39 had a much higher speed than the B-29, which would have made its use favourable had WWII went on or led to an inter-Allied conflict. It might have become the USAAF standard big ship of 1946/47.