Internally, that was more from limit on existin number of mount points on the bay, in 1938 the very heavy(2000+GP bombs) were not common
It had a lot of payload, and had large number of internal tankage 10500 gallons. as far as speed, I don't think the external shackles were ever used. It just din't have enough HP
It had enough wingspan for six engines. the BV-238 Seaplane had six engines on a 197 foot wingspan, vs 212 for the XB-19
Had the liquid cooled V-3420 engines been ready in 1938, the B-19 could have had pushers like the B-36, without the difficulties pf cooling radial Wasp Majors