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Order Consolidated to build B-17s. Just as RLM ordered Arado, Henschel and Heinkel to participate in the massive Ju-88 light bomber program during 1938. Otherwise Consolidated is cut off from government funding until they go bankrupt.
B-17 first flight was 28 Jul 1935.
B-24 first flight was 29 Dec 1939.
B-29 first flight was 21 Sep 1942.
IMO the B-17 could get the job done until the much larger B-29 is ready.
It appears to me the 1941 B-17E was the last model to receive major design changes. After that it was a matter of cramming more gun turrets on the same airframe powered by the same rather small 1,200 hp engines.
Without the huge B-24 program the B-17 might receive additional development. Start by considering a more powerful engine to increase the payload. By 1942 1,200 hp was rather small compared to engines which powered British and German bombers.
The 1944 Fighter Conference took place in October and if one looks at the way the P38 placed in the different categories it is apparent that the pilots, not the leadership, overall were not impressed with it.
Also, two engines adds cost, reduces mission reliability and increases maintainability requirements. Faced with the simplicity, cost, ease of flying and performance of the P-51, it was difficult to justify.
On the other hand,in either the South Pacific or the Indo-China theater they were known to return home flying on one engine for 600 miles. I am not sure about mission reliability but any pilot who made it back to base flying hundreds of miles across open water or SE asia jungles probably wasn't complaining about cost or maintenance requirements
The P-38 received a bunch of reliability and ergonomic improvements between 1941 and 1944. They add up to a much more effective combat aircraft even without considering more powerful engines and power assisted ailerons.
Improved cockpit heater.
Improved intercoolers.
Improved turbocharger controls.
More efficient radiators.
Improved electrical system.
"Combat Maneuver" setting added to the Fowler flaps.
Drop tank supply caught up with demand.
Improved radio.
Improved 20mm cannon.
Improved hard points for mounting bombs and drop tanks.
Internal fuel increased by adding tanks in wing leading edge.