I'm not sure that would be a successful gambit in a real war. I would point to RMS Lusitania.
KAL-007.
Even Clancy had the “look like a civilian airliner” trick in Hunt for Red October. At anything beyond visual range a B-52 is indistinguishable from a 747 or other large airliner.
Bingo. Good historical reference. Can’t distinguish it from a civilian ship; then assume its military.
In a 'real war', civilian airliners will stay away from the war zone, if they have any sense. This would launch its missiles from up to 1000 to 1500 miles from the target. No enemy SAMs or interceptors can cover that large an envelope.
If you are really worried about interceptors, mix in a few "P-51 Mustang" drone missiles to provide 'escort' if needed. For that matter, the Phoenix system could track enemy fighters out to 120 miles and engage them 60 miles away. The upstairs deck of the 747 has plenty of room for Weapons Intercept and Electronic Countermeasure Operators!
No civilian aircraft in wartime. They become part of the reserve fleet. That’s how we were going to reinforce Europe during a war.
In a real war, I wouldn’t be flying around unless it was directly related to the war effort. I think tourism would be put on hold.