Bombers designed in the '30s were faster than fighters designed in the '20s and early 30's and they were as fast as fighters designed late in the 30's. In that particular time more time was spent developing bombers specifically to be faster than the fighters. Problem with that is 1. the bombers didnt carry a large load 1,000 to 2,000 lbs of bombs, 2. As soon as the bombers got faster, fighter development picked back up and soon the fighters were faster than the bombers. As soon as they figured out that the bombers needed to carry larger loads they realized that they would not be able to outrun the fighters so they had to arm them, and then add armor to protect the crews. Yes adding armaments to the bombers slowed them down, but by the 40's any bomber carrying a significant load of bombs was not going to be able to outrun a fighter anyway and they needed the guns to defend themselves. There were exceptions like the Mosquito but it only carried 2,000 lbs and then later 4,000lbs. It wasnt so much the guns themselves but the defensive formations were the bombers flew close together to provide support to each other and provide a deterient to fighters. You dont have to shoot down a fighter, just tear it up, hole its radiator, damage the wings, wound the pilot bad enough and they'll go away
It would be interesting to know how fast a B-17 would be with only 4 crewmembers, no guns and ammo and the extra oxygen, etc and only 8,000lbs of bombs