Depending on the perk multiplier can yield some good perks in a 29 though especially if you get a few kills. Think the highest ive ever earned in 29's was around 15 perks.
The perk bonus modifier must be quite good to get thta many perks. And even then, A2A kills won't contribute much to it when flying the B-29. You'd need a perk bonus modifier of 5 just to get a single perk out of killing a Ta 152H.
That being said, my maximum perk run in the B-29 was this one:
08-23-13
map: compello
target faction: Bishops
type: strat attack
takeoff time 11:11 CEST
fuel 75%, bombs 8x2k
bombing altitude 27.5
total flight time 1:43
destruction: 26xaaf, 24xamf, 18xbaf, 31xrdf, 2xgng
perks earned: 18.4landed damage displayed: 23,350
score points: 1,007,268
kills: none
deaths: none
uneventful flight, no interceptors
But overall, I had a net loss of more than 6000 perks flying B-29s since September 12