There's been a few proposals over the years to do just that, but they haven't worked out mostly because the Air Force wasn't looking for a new bomber at the time and/or already had enough bombers to do the mission. It's unlikely that the Air Force would have ever taken a civilian plane as a bomber though. While there are some civilian derived planes in use, like the KC-46, P-8, smaller cargo, VIP transport, EWAR aircraft, and in other countries AWACS planes, all of those are designed to be used in relatively safe areas. A bomber on the other hand should be able to fly in contested airspace, possibly at low altitude at high speed, with intensive maneuvering, and a whole bevy of sensors and jammers to stay alive. Plus, modern bombers need to be stealth as well, which isn't something that can be retrofitted to an existing design