https://oldmachinepress.com/2017/04/20/allison-v-3420-24-cylinder-aircraft-engine/
Here is a link to a quite extensive article describing Allisons' experiences in developing the V-3420. To quickly summarize the V-3420 was a sound and reliable design that suffered mainly from poor company management and redirected priorities. It was a missed opportunity.
Of course once the Army Airforce settled on the Wright R-3350 then every effort and resource had to be made to correct the numerous flaws in its design. It took years to get the R-3350 working reliably. And eventually they went to a better engine for the B-50 anyway. As
@Dynasoar points out why didn't the Army Airforce select a better radial engine for the B-29 in the first place?
The Allison V-3420 would have been a great engine for the B-29 if the design and development team at Allison had received even a fraction of the money and support misdirected on the R-3350. The V-3420 worked quite well on the B-19 and also on the XB-39 even without the turbosuperchargers installed due to another V-3420 delay one can attribute to the lack of love for inline bomber engines in the USAAF.
Another advantage if the V-3420 had been chosen for the B-29 program is the radiator housing design could have utilized the Meredith effect is a similar way as was done with the P-51. An important benefit for an airplane that spends many hours flying at a high cruise speed.