B-24s seem to be exceptionally durable, yet they are no more durable than the Lancaster.
Here's what I did. I went offline and practiced killing the B-24 drone, attacking from various angles. I found that a short burst is enough to bring one down if you put the rounds where they count most.
Aim for the wing between either inboard engine and the fuselage. This is almost certain to start a fuel fire. Once it's burning you need only wait for the explosion.
Another area is the cockpit. 50% of the time, a good hit on the cockpit blows up the Liberator.
When possible I like to attack from directly above.
My regards,
Widewing