Okay, a brief as I can make it history of how and to some extent why the Whirlwind ever came into existence.
Specification F.37/35 was issued by the Air Ministry on 30 March 1936 and Armstrong-Whitworth, Fairey, Hawker, Vickers and Westland were invited to submit proposals by 1 May. Although the specification was originally for a single engine type there were serious doubts about mounting larger calibre weapons in the wings of unbraced monoplanes and the specification was modified to include twin-engine types.
Of those invited, only Hawker and Westland submitted proposals. In addition Bristol, Boulton Paul and Supermarine asked and were allowed to tender. These five companies proposed a total of eight designs.
Boulton Paul offered two single-engine monoplanes with wing mounted cannon. The P.88 'Scheme A' was powered by a Bristol Hercules, 'Scheme B' by a Rolls Royce Vulture.
Hawker submitted a version of the Hurricane with two Oerlikon cannon in each wing, even though the Air Ministry had expressed a preference for the Hispano. One wonders how serious they were.
Bristol submitted both a single-engine and twin proposal. The Type 153 was powered by a single Hercules, the Type 153 A by two Aquilas, the smallest of the companies sleeve valve engines.
Like Bristol, and in search of the most compact design for their twin-engine submissions, Westland and Supermarine both specified the smallest possible engines consistent with achieving the required performance. Supermarine's Type 313 used the 21 litre Goshawk B, and evaporatively cooled derivative of the Kestrel and Westland submitted their P.9 also powered by a pair of Kestrels.
Supermarine also submitted the Type 312, a single-engine proposal with wing mounted cannon which was clearly a derivative of their Spitfire.
At the tender design conference in May 1936 The Vulture powered Boulton Paul P.88 B was selected as the preferred single-engine design, followed by the Bristol and then Supermarine and Hawker designs.
The Supermarine Type 313 was the preferred twin, though Verney though the Westland P.9 was a more advanced design. Another issue was Supermarine's inability to deliver quickly. The company had estimated 27 months for the twin design, though Westland was not much better, estimating 18-24 months.
Wilfrid Freeman felt that some degree of competition was needed and on 24 August a single prototype was ordered from Westland. Without Freeman's desire to 'encourage' Supermarine the Whirlwind would never have made it from a paper project. Subsequently, Supermarine advised the Ministry that their experimental department was so busy that they would prefer to build just one prototype. This was agreed, one prototype was ordered from Supermarine and two from Westland.
The Whirlwind had had its first lucky escape!
Boulton Paul quoted £38,000 for its two prototypes, Westland £45,000 and Supermarine £23,361. This far exceeded the budget set aside for the programme (just £20,000) which sent the Air Ministry scuttling, cap in hand, to the Treasury. The funds were forthcoming and on 7 December the Ministry ordered the two Boulton Paul prototypes and the single Supermarine Type 313 but NOT the Westland prototypes.
The Whirlwind looked dead in the water.
Why was the order not made? It was due to financial/merger shenanigans at Westland which had caused several senior figures at the company to threaten resignation. These included technical director Teddy Petter, factory manager John Fearn and the entire board of directors, with the exception of Sir Ernest Petter himself. The Ministry estimated that without Fearn and Teddy Petter the delivery of Lysanders would 'probably be delayed by 6-9 months' and that there would be no point in ordering the F.37/35 prototype which design, according to Newall, existed only in Petter's head. Sir Ernest was summoned to the Air Ministry and told in no uncertain terms that unless Westland kept its existing design and production staff there would be no more Ministry orders. On this understanding the Westland prototypes were ordered on 19 January, though the contract had clauses allowing the Ministry to cancel, without penalties, if Fearn or Petter were to leave the company.
Lazarus had risen again.
Then, on 28 January the Supermarine prototypes were cancelled to allow the company to concentrate on its bomber to B.12/36. The Westland P.9 and single-engine Boulton Paul P.88 were now the only contenders in a two horse race.
Initially Westland made good progress with their prototypes, so much so that by October 1937 the Air Ministry was questioning the wisdom of continuing with the Boulton Paul P.88. In May the Ministry had issued F.11/37 for a two seat cannon turret fighter, for which Boulton Pauls P.92 seemed the most promising candidate. Their Defiant project was running behind schedule and progress on their F.37/35 prototype had been slow. Worried that ordering the two cannon fighter prototypes was over stressing the company's resources, on 19 October the Ministry cancelled the P.88 and ordered two P.92 turret fighter prototypes.
This left the Westland P.9 (Whirlwind) as the only remaining contender for the F.37/35 project. And that's how an aeroplane which was initially not a serious contender emerged from the back field to be the last man standing.
How the wheels subsequently fell off, and how an aeroplane ordered in 1936 did not have a production example flying until May 1940, delivered to the RAF in June, is another story.