So, obviously, I missed the
U.S. Enters WWII Early (1940) GB deadline on this one ...
Lockheed's 'Big Twin' - the FM-4 PoleStar Long-range FighterBack in June 1936, Lockheed had submitted its Model 11 to meet the Air Corps' Specification X-604 for a multi-place fighter. Initially given the military designation XPB-3 (Experiment Pursuit, Biplace 3), the Model 11 had its origin in the Model 10D - a proposed military derivative of Lockheed's
Electra airliner. But the Model 10 airframe was developed out of all recognition. The radial engines were swapped for Allison V-12, a tricycle gear adopted, the cockpit flanked by two powered turrets armed with 37 mm Browning T9 autocannons.
Under the Air Corps' points system, the Lockheed Model 11 - by then re-designated XFM-2 (Experiment Fighter, Multiplace 2) - did well. Alas, the even-more-radical Bell XFM-1
Airacuda scored slightly better. Despite its high test scores, the
Airacuda as built proved overly complex and prone to systems failures. In any case, the overall concept of the multi-place 'cruiser' fighter was waning in planning circles. Still, the United States mainland needed protection from long-range bombers. As a result, the Department of War had approved Specification X-608 for a 'Interceptor Pursuit (Twin-engine)'. This contest was won by 'Kelly' Johnson's ambitious Lockheed Model 22 which became the XP-38
Lightning.
Air Corps planners were still looking for an even longer-range 'Mid-Atlantic' interceptor. Internally, this requirement was referred to as a 'PB' (Pursuit, Biplace). However, when Specification X-610 was issued, it called for a 'Fighter, Multiplace (Twin-engine)' which was to be based on a proven airframe. In other words, the USAAC was looking for an improved 'FM' design. The X-610 requirements reflected a critique of the service trial Bell YFM-1 (and, by extension, the Lockheed XFM-2 concept). X-610 specified that barbettes and turrets were to be avoided in favour of a simpler fixed-gun main armament.
To meet Specification X-610, Bell submitted a simplified, tractor-engined
Airacuda derivative. [1] In turn, Lockheed submitted two 'Kelly' Johnson designs - the Model 22FM (based on the
Lightning) and the larger Model 35 (which could be seen as a spin-off from the earlier Model 11/XFM-2). Both Lockheed submissions were intended as 2-seaters but the Model 35 had the option of a third crew member. Compared with the Model 22FM (and the earlier Model 11), the Model 35 was a conservative design - combining features of the Model 10 and the bigger Model 14 airliner airframes.
The Model 35 was, effectively, a higher-powered Model 10
Electra with a refined fuselage. The design approach reflected a critique of the earlier XFM-2 concept. A simpler fixed armament was specified by X-610 ... so the Model 11's bulky turrets disappeared. For the extended, over-water operations now envisioned for the 'FM' role, 'Kelly' Johnson decided to abandon the Model 11's Allison inlines - for fear of combat damage to vulnerable cooling systems. In place of the V-12s were bulkier but highly durable Pratt & Whitney R-1830
Twin Wasp radial engines.
At a glance, the Model 35 fuslage looked very similar to that of the Model 10 airliner. In fact, it was completely different - being of a nearly triangular section. This layout was adopted to provide plenty of room to the sides of the cockpit to position to specified twin fixed, forward-firing 37 mm Oldsmobile M4 autocannons. Above and forward of each cannon breech was mounted four .50-calibre heavy machine guns. A fifth .50-calibre was to be mounted as a remotely-fired 'sting' gun in the extreme tail.
Top The prototype Lockheed Model 35 as originally flown. Note the unfaired main undercarriage bays and small 'bubble' glazing for the observer/navigator. The windows let into the sides of the fuselage are to illuminate the 37 mm gun breeches - should the nav/obs need to clear blockages. This aircraft was actually unarmed (the visible tail 'sting' gun is a dummy barrel).
(To be continued ...)
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[1] This tractor-engined
Airacuda development was never built. Nevertheless, Bell was assigned an Air Corps designation for its design - XFM-3.