Don't forget the Halifax!
The way I see it:
Unescorted Day Bomber: B-17 (better ceiling and defensive firepower than Lanc/Halifax, better survivability than B-24)
Escorted Day Bomber: Lancaster (better bombload, faster cruising speed than either B-17 or B-24)
Night Bomber: Lancaster
Maratime Patrol Bomber: Halifax III or PB4Y-2