The A-26 entered combat service Nov 1944 in Europe and Jan 1945 in the Pac. My uncle Ed's B-25 Sweet Adeline was replaced by the Invader in Italy.
If you want to compare the B-17 with Cyclone and R-2800, you could compare the DC4 and the DC-6.
North American had a plan to build a B-25 powered by R-2800, and were told there was not enough production. They jury-rigged one up anyway, and told the pilot not to exceed a certain amount of Gs because the R-2800s had more power and weight than the R-2600s and the wing wasn't strengthened. It was such fun to fly that it inspired excess. There's words for his tombstone.
The difference between the two cargo planes is insane. Almost 100 mph cruising speed and around 1/3 more max load.
The B-17 came anywhere close to that and you are looking at an aircraft as fast as the B-29 top end and enough extra lift to not only handle all the bits and pieces that made up the YB-40, but a usable 10,000 pound bomb load clear to Berlin and back.
Yikes!