In article , The Revolution Will Not
Be Televised writes
They evolved that way, from roughly similar airframes as a starting
point. Lancasters flying by day would soon develop heavier armour,
especially around the engines, less bombload in exchange for more fuel
to burn for higher height on the ingress route, and heavier armament
like .50 calibres in the rear turret - all of which they were adopting
by 1945, which cut into their bombload margin over the B-17.

Interesting point, any sources for this. I haven't heard about
increasing armour for daylight ops, or trading bomb load for fuel. The
B1 Specials had virtually everything not nailed down stripped out, lost
their armour and most of their guns. The Lanc achieved its greatest
bombload in 1944-5 by daylight.

The 50s in the rear turrets were IIRC fitted only as a pair instead of
the quad 303s.

--
John