Why the big change in aircraft production 1939 to 1941?
War.
I know that sounds over-simplified, but it in 1939 was obvious that the world was at war again, and even though the war was in Europe, it still had to be considered as a threat to US security, to the security of the western hemisphere as defined by the policies of the Monroe Doctrine.
Why make Aircraft
A book from 1948, The Army Air Forces in World War II: Plans and early operations, January 1939 to August 1942, published by the Office of Air Force History has quite a bit to say about this development phase, and the reasons why it took pace. (all following emphasis mine, web page for the information here
When war broke out in Europe in September 1939, the United States
declared its neutrality.4 Its stated policy was to remain out of the
conflict if possible and at the same time to keep the totalitarian
powers out of the Western Hemisphere. The latter objective demanded a
further extension and an acceleration of previous programs for
strengthening American armed forces. It called also, because of the
threat of new weapons and modes of warfare, for the establishment of
new strategic bases. The declared policy was wholly consonant with the
Monroe Doctrine, and the measures taken to enforce it were, for a
while, consistent with our traditional ideas of defense.
From Sept 1939 on, there was an understanding that, even if we could maintain neutrality, air power was going to play a strong role in this new conflict. We had the advantage of observing this, and reacted accordingly.
The new concept was influenced by the military techniques of the Nazis
as well as by their unbroken string of victories. The pattern of
political infiltration, violent air attack, and machine-like
blitzkrieg encouraged the conviction that defense of the Western
Hemisphere was closely linked with the survival of the Allies,
especially of England: an Allied victory would forestall an Axis
invasion of the Americas and even by merely prolonging Allied
resistance the United States would gain time needed for building its
defenses. So in 1940 aid to the Allies had become the avowed policy of
the American government.
So, we see an understanding that war could spill over to this hemisphere, and that the best defense we had at the time was the survival of the UK. Steps were taken to develop a strategy to maintain our security:
Preparations initiated by the United States government in the period
1939-41 involved, then, four interrelated activities: expansion of the
military establishment; development of a new doctrine of hemisphere
defense; aid to the Allies; and the formulation of strategic war
plans. To an important degree the nature of those defense measures was
determined by the nature of the conflict in Europe. Thus in light of
the vital role played by the air arm in German offense and in British
defense, it was natural that the United States should place great
emphasis on the development of air power. And hence during the period
1939-41 the Air Corps figured prominently in each of the four aspects
During the time of the buildup in this question, the Battle of Britain takes place. This put any doubts concerning the necessity of advancing the concept of air power as being vital to a nations defense. The UK was fighting for its very existence, and there were no ships, no tanks, no infantry soldiers involved in the fight (metaphorically speaking). Air Power.
Now since the title line question asks why this occurred, I'll stop here. The cited book does continue with much more information concerning how this expansion took place. In the desire to not copy an entire chapter from a book, I'll stop here for now.
How did we increase Production so quickly?
Ok. A little on the how, and the numbers involved (still from the same source above)
The presidential message to Congress of 12 January 1939 marked the
beginning of a period of Air Corps expansion which did not reach its
peak until 1944. Asserting that "increased range, increased speed,
increased capacity of airplanes abroad have changed our requirement
for defensive action," President Roosevelt strongly urged that
$300,000,000 be appropriated for the purchase of aircraft for the
Army.6 The existing force, which the President described as "utterly
inadequate," consisted of approximately 1,700 tactical and training
planes, some 1,600 Air Corps officers, and 18,000 enlisted men.7
Within three months Congress had passed an emergency Army air defense
bill substantially as requested, authorizing the procurement of 3,251
aircraft. This act approved a total Air Corps strength of 5,500
planes, 3,203 officers, and 45,000 enlisted men. The appropriation
amounted to half as much as the Air Corps had received in the fourteen
preceding fiscal years;8 approved strength for officers was doubled,
for enlisted men was increased by 150 per cent.
So, we can see the need was recognized as early as 1939, and authorization for a moderate number of aircraft was given. One reason production was able to tool up so quickly, however is mentioned here:
Because the Air Corps had anticipated these authorizations, there was
little delay in inaugurating its expansion program. As early as autumn
of 1938 the Chief of the Air Corps had asked American aircraft
manufacturers to prepare for an unprecedented growth, though no orders
could then be guaranteed to them.9 By the time the new appropriations
had been approved, contracts were being negotiated and tooling-up had
begun.
So preparations were begun before the authorizations actually cleared congress. The direct results of the conflict over Britain can be seen later in the changes to the numbers of planes authorized:
To take advantage of approved increases in aircraft and personnel, the
Air Corps in the spring of 1939 formulated a plan calling for 24
tactical groups to be combat-ready by 30 June 1941.10 Long before this
objective was reached, however, the trend of events abroad urged
further expansion, and in May 1940 the Air Corps projected the
41-group program.11 Within two months the goal was again revised
upward in the 54-group program, which would provide an air force of
4,00 tactical planes, 187,000 enlisted men, 15,000 aviation cadets,
and 16,800 officers.12 In autumn of 1941, the Army Air Forces, in
anticipation of the vast expansion contemplated in the as yet
unapproved Victory Program for munitions, formulated the 84-group
program, which would enlarge the force to a personnel strength of
400,000 by 30 June 1942.13
You can see we started requesting the formation of 24 tactical groups, but in the summer of 1940 this changed, to 41 and then 54 desired air groups. You can see the direct effect the Battle of Britain was having.
Besides the why we needed these aircraft question, a point of interest is How did production ramp up so quickly
The aspect of expansion which was most eagerly followed by the
American public was the rapid growth in the output of military
aircraft. The seemingly miraculous accomplishments in this respect
were essentially civilian rather than military. they were made
possible through the fortunate combination of a highly adaptable
industry, great national resources, and generous government aid; but
the Air Corps played here a significant, if lesser, part. Contracts in
1939 were awarded on the basis of single-shift production, but
factories moved steadily into a two-, then a three-shift schedule as
more trained personnel became available.15 The Air Corps, along with
the Office of Production Management, adopted carious methods of
acquainting manufacturers with new types of aeronautical equipment, of
spreading production among more firms, and of increasing the capacity
of the industry. "Educational orders" were placed with manufacturers,
existing facilities were enlarged by the aid of government financing,
and new plants were built by the government for operation by private
firms. "Letter contracts" saved from one to three months in initiating
the fabrication of aircraft by permitting manufacturers to purchase
materials before a formal contract could be drawn up and signed.
Competitive bidding was supplemented by the more rapid procedure of
direct negotiation of contracts. In general, the Air Corps profited by
its long and intimate association with the aircraft industry. Friendly
personal relations made for mutual confidence, and reliance upon a
telephone conversation or a quick airplane trip frequently obtained
results which would have required weeks of formal correspondence. Some
of the new methods were more expensive than the traditional ones, but
by 1940 the nation, though it is now easily forgotten, had more money
than time. In some ways the Air Corps was able to exert a direct
influence on speeding up production. Standardization of aeronautical
equipment was one method. This subject had been under study by the
Army and Navy for more than a decade. Joint efforts toward
standardization had begun with such minor items as nuts, bolts, and
pressure pumps. By the time rearmament began the Army and Navy were
procuring aircraft engines from two major contractors under terms
which made most models equally acceptable to either service. Further
efforts were made toward standardization of aircraft and related
materiel used in common by the U.S. and British services. Such a
policy was advantageous to both manufacturers and purchasers; it
facilitated mass production, lessened confusion for the producer, and
reduced overhead costs. Again, the Air Corps was able to reduce the
long delays usually experienced in testing new models for acceptance.
An accelerated service-test procedure was instituted whereby
experienced crews in relays gave an airplane 150 hours of almost
continuous flight with a full military load. This system brought to
light i one month defects which formerly might have required a year of
service testing for discovery.16
So these are some of the processes which allowed the increased rates in production. Some questions concerning which service branch these planes were targeted for is also addressed here:
These speed-up measures had begun with the initial appropriations for
Air Corps expansion and had been intensified as U.S. and British
demands increased. The 1939 Air Corps objective of 5,500 planes was
soon raised to 10,000. Then on 16 May 1940, with the extension of the
war in western Europe, the President called for an annual output of
50,000 aircraft and a total Army and Navy strength of the same number
of planes; approved figures provided 36,500 for the Army, 13,500 for
the Navy.