The Whitley was sort of in a class of it's own when it came to bomb load and range, Due in large part to fact that it was in a class of it's own when it came to speed.
A big wing and a slow cruising speed (economical was 135mph? and max lean about 180mph? making it 25-45mph slower than most of the other older bombers) make for good payload and range but a plane that needs to fly at night to have a hope of surviving.
The Whitley was often referred to as a "heavy" bomber and not a medium and was intended from day #1 to operate at night.
SR6 yes i thinked that the long range of Whitley came out from the low speed, for this i've ask for 111 cruise data
and the data uploaded from Dennis show as the 111 going slower can go farther