So, improving the Whitley could be done, but John Lloyd has to be outed as the designer. The Whitley was based on the AW.23 bomber transport, which the former looked like a mini-me version of the latter and although the Whitley had an entirely new fuselage of different construction, the basics of the wing remained the same. The Whitley's wing was thick because its predecessor's was too and it was a characteristic of the family, the Ensign and Atalanta shared a similar profile. The wings had a box spar of corrugated panels with ali formers forward of the box section covered with sheet ali and to the rear of the box section covered in fabric. Its thickness benefitted the Whitley for two reasons, the fuel tanks were in the leading edges and between the nacelles and the fuselage it had wing bomb cells. The design was rigid and very strong.
AW.23 model. Only one of the type was built.
MAM 17
That can easily be changed. A thinner wing with fuel cells within the box spar design, and make the whole thing stressed skin, like the fuselage; no fabric trailing edge. The fuselage should be redesigned with the wing set higher to enlarge the bomb bay below it, the Whitley's bomb bay was interrupted by the main spar, which meant there was little flexibility in the size of bombs it could carry - it was most definitely a product of pre-war thinking with no room for growth, so give it a big unobstructed bay like that on the Manchester. To keep the fuselage cross-section down, don't incorporate a transport requirement into its design. It wasn't very big inside the Whitley's fuselage, but there was the capacity to carry troops, which it did on Special Operations. The problem is, most bombers of the pre-war era, including the Stirling to B.12/36 and the Manchester and HP.56 to P.13/36 had a troop-carrying role. Get rid of it!
That big 33-foot long bomb bay in the Manchester was unique, no other bomber had such a big unobstructed space - it was carried through unaltered in size into the Lancaster.
Lancaster bomb bay
Engines as per your suggestions regarding the types of Merlin. The one good thing the Tiger engined Whitley introduced was a de Havilland two-position variable pitch prop, so keep that.
Defensive armament, the Whitley was always intended on having turrets and its first were the Armstrong Whitworth manually operated turrets, as when the Whitley first flew in 1936, Nash & Thompson had not built a useable fully enclosed power turret. Frazer Nash had developed hydraulic operating gear for the Handley Page Harrow's defensive guns, but they were not in a self-contained turret, the Whitley had to wait until the Nash & Thompson four-gun FN.4A rear turret in the Whitley IV and that didn't appear until 1938, but it still had an AW nose turret. The Whitley V was similarly armed but had a rear fuselage extension, which enabled a wider arc of fire for the rear turret and the unpowered turret in the nose was eventually swapped with an FN.16 nose turret, so powered turret armament in our 'new' Whitley, too.
Armstrong Whitworth unpowered turret with its single Vickers machine gun. The early marks of Whitley originally had two of these as their sole armament, but the Mk.III also had a ventral gun position with two machine guns.
YAM 59
Whitley rear fuselage section with Nash & Thompson FN.4A rear turret. With the introduction of the Mk.IV, the Whitley had a rear gun armament with firepower greater than any other bomber gun installation in the world. No other bomber built anywhere else could match four machine guns in a power turret.
Royal Museum 26