Monogram opted not to include bomb bay detail, when they developed the masters and the molds for the B-17, to keep costs down, while producing a model that otherwise met their standard for detail. Remember that that kit goes back to the early 70s, and at the time it was developed, it was one of the best model kits we had ever seen.
Revell opted not to include the detail, because they copied the Monogram kit for their B-17F, and they wanted to rush it to market. Monogram's executives sent them a letter, outlining point by point the details that proved it was a copy, and telling them that they wouldn't sue, this time, but if it ever happened again, they would.
If you're interested, I recommend Thomas Graham's books on Monogram and Revell, on the history of each company, they make for enjoyable reading, especially for us old-timers who built those kits as kids.
Hope that helps!
Regards,
Brad