One popular method I have seen is to use FS Recorder. You can record a flight with a B-17 and then play that back as traffic and escort with the P-51. This requires you to fly both planes at different times. You can then re-record your flight escorting the B-17, play that back and now you can fly as a wingman for the first P-51 escort. It is labor intensive and you have to coordinate carefully but it can be a lot of fun
This video was made using FS Recorder as stated above. You can then make a formation of B-17s by within FS Recorder copying that B-17 as many times as you like and give each aircraft offsets from the original. You can then fly your P-51 with the B-17 formation. The video was made by flying 1 x B-17 then 1 x P47. The rest were copies. You don'r have to give the 150 aircraft offsets. I did 18 B17s in an authentic 1943 formation (from a book) then saved that file, opened that file a few times in FS Recorder playback and give each of the play back formations of 18 aircraft a time delay of say 10 seconds for the first 18 aircraft formation, 20 seconds for the second and so on. How many aircraft depends on your system. Mine is not the best.
It is best to get the first aircraft into your start point, pause the sim and save the flight in FS. This makes it easier to get your P51 near the bombers to start with when you play back your formation. Maybe turn Crash detect off until you get away from the bombers otherwise you may collide.
If anybody would like my B17 formation offsets reply here and and I will post.
EDIT "If you want to fly with a larger formation, start recording your flight in FSR on a TRUE North heading, even if its only for a couple of seconds. This makes it easier allocating the offsets as they are North and East distances. You can then change to a suitable course."http://youtu.be/0n0_e3ILqtE?hd=1