Sorry for the consecutive posts but my "why" question on the Manchester's requirement to carry torpedoes led me to a internet search and the following extract raises as many questions as it answers. It seems to be a case of trying to get the plane type to do as many roles as possible (sounds familiar?)
I would put the requirement for the Manchester to be stressed for catapulting and capable of carrying a torpedo in the same disaster area as the requirement for the He 177 to dive bomb.
Happily both requirements were relaxed for the British plane but the large bomb bay remained as a benefit in the Manchester and the Halifax.
The largest torpedo capable WW2 plane I can think of was the Martin Marauder with an empty weight of about 2/3s of the Manchester, the Betty and the He 111 were much lighter.
A feature of the 1936 bomber specifications that had unlooked for beneficial repercussions was the requirement that the medium bomber P. 13/36 should be capable of modification to carry torpedoes. When tenders to specification P. 13/36 were received, it was found that provision to carry two 18 in. torpedoes (which were 18 ft long) without altering the main structure of the aircraft, or losing performance, was causing design difficulties. This led the DDOR (Oxland) to review the discussion on this issue that had taken place at the two Operational Requirements Committee meetings on P.13/36. He told the DCAS (Peirse) that the Coastal Command representative had given his C-in-C’s view that the aircraft was too large and expensive for a torpedo bomber. Even so, the then DCAS (Courtney) had argued that whilst there was the possibility of a limitation on the numbers of first-line aircraft, it was desirable that every unit should be as effective in war as possible. As we have seen, a muted form of the torpedo-carrying requirement was therefore included in the requirements.
Oxland recommended that as a dedicated torpedo bomber was now under development (B.10/36 — Bristol Beaufort) the torpedo requirement should be deleted from P.13/36. Alternatively, he said, provision could be made for a limited number of the aircraft to have larger bomb doors and so on.
The Operations and Plans branches of the Air Staff did not agree with deletion of the torpedo requirement. They advised Peirse that the Admiralty had yet to be persuaded that, ‘the “B” bomb is in every way a more efficient weapon with which to attack ships’, and that a torpedo bomber version of P. 13/36 should be developed until the Admiralty was convinced otherwise. (The ‘B’ bomb was designed to be dropped in the path of a ship, sink, and then rise to strike the bottom of the ship as it passed over.)
This discussion was made redundant when the Operational Requirements branch announced that it had new information on the size of torpedoes. It now found that the Avro P. 13/36 could carry only one internally, and that in any case existing torpedoes could not be released at 150 mph from 200 ft. If this was relevant, the DCAS (Peirse) must have wondered how the idea of torpedo carrying for P.13/36 had arisen in the first place. It appears that the Operational Requirements branch had given no more thought to the operational problems of torpedo dropping than they were later found to have given to catapult launching. Peirse decided that torpedo carrying would no longer be asked of the P.13/36 bombers. Nevertheless, the long bomb bay that had been required was to prove valuable when bombs larger than the 2,000 lb were found to be needed.