A Wellington Mk I or Mk III would be 1) more representative of RAF Bomber Command prior to the Lancaster/Halifax, 2) less work to add and 3) work in settings where we currently lack an RAF bomber.
I'd like the Wellington B.Mk III. Built using a tough, geodesic construction, defended by eight .303s, two in the nose turret, one in each beam and four in the tail turret. It carried 4,500lbs, so a useful war load in the Early War arena and settings. With a top speed of about 240mph it would also be interceptable by early war Luftwaffe fighters, something the Boston Mk III really isn't.
I advocate adding the Mk III instead of the Mk IC due to the quad .303 turret. I don't think a 240mph bomber with only two .303s in the tail would be viable, gameplaywise, even against Bf109Es, Bf109Fs and Bf110Cs.
Direct hit from flak:
According to the note, this bomber was hit before it reached its target, despite the damage it completed its run and returned home to a safe landing. The tail gunner, obviously, didn't make it. After completing the bomb run they found they could not close the bomb bay doors due to a loss of hydraulic power.