I saw add a small perk cost to the Lancaster, B24, B17, and maybe even the B26 and Tu2.
Stop and think of the destruction a flight of them can bring to the table. The speed demons get a perk for not able to be intercepted (Ar234's, Mossi B Mk 16's), and the B29 gets a perk because of the massive amount of destruction, long range, defensive capability, and difficulty in being intercepted (speed).
Why not apply a 5 perk cost to Lancs? 4 to B24's, 3 to B17's, and 2 to the B26 and Tu2??? It isn't like bomber perks are hard to earn.