American Warplanes – Interwar Aircraft (1919–1938)

Upon the conclusion of the First World War in November 1918, the Air Service had in its possession approximately 7,900 aircraft, of which less than 1,000 had seen operational service in France. The majority of the wartime-acquired planes would be sold as surplus, with some placed into storage. All existing factory contracts for new aircraft were cancelled.

In June 1920 the Air Service was established as a combat arm of the US army, on a par with the other branches of the service. In July 1926 the US Army Air Service was re-designated as the ‘US Army Air Corps’ and authorized 1,800 aircraft.

We need bombers

The leadership of the Air Service had been very impressed by the wartime development of foreign multi-engine bombers intended for the strategic bombing role, so much so that it had arranged to have modified examples of both an Italian three-engine bomber named the Standard Caproni and a twin-engine British bomber named the Handley Page built in the United States by the New Standard Aircraft Company. Due to a number of delays and the ending of the First World War, only five units of the Italian bomber entered service with the Air Service post-war.

The Air Service also sought out a native-designed multi-engine bomber. This prompted them to order from the Glenn L. Martin Company a twin-engine biplane bomber designated the MB-1 in late 1918. None would be completed in time to see combat in France, and after the initial ten units were completed, further production was cancelled.

The Air Service contracted with Martin in 1921 to produce twenty units of an updated version of their MB-1 bomber, to be known as the MB-2. The first five built retained the company designation MB-2, with the following fifteen being referred to as the Night Bomber, Short Range (NBS-1). It was felt that the slow speed of these bombers would make them easy targets for enemy fighters during daylight hours.

This initial order for the MB-2/NBS-1 was followed by another from the War Department for an additional 110 units of the aircraft. As the War Department competitively bid this order, Martin was underbid by other companies for the building of the aircraft which remained in use until the late 1920s.

Post-war military aircraft designation codes

Prior to 1920, American military planes typically retained their builder’s designations. Beginning in 1920, the letter/s acronyms at the start of an aircraft’s designation indicated the aircraft type and size. The trailing number/s would denote the chronological sequence in which the plane was taken into service. Suffix letters in alphabetical order following the designation number/s of an aircraft would identify variations to the plane.

In 1923 the Air Service acquired a single-engine biplane light bomber designed and built by the Huff-Daland Company. It was designated the Light Bomber No. 1 (LB-1). As the Air Corps had decided in 1926 that it only wanted twin-engine bombers, only nine units were taken into service.

The Huff-Daland Company soon came up with a new twin-engine biplane bomber design. The Air Corps liked what they saw and ordered ten units in 1927 from the Keystone Aircraft Corporation, successor to the Huff-Daland Company.

The new Keystone bomber was labelled the Light Bomber No. 5 (LB-5). Upgraded versions were designated the LB-5A, LB-6, LB-7 and LB-10. They would serve until 1934. From 1930 onwards, all bombers were identified only by the letter ‘B’.

Next-generation bombers

The Keystone twin-engine biplane bombers were merely minor updates of the First World War designs, built mostly of wood and fabric. They also had open cockpits, fixed landing gear, and their bombs were carried below their wings as they had no internal bomb bays.

The first somewhat modern bomber tested by the Air Corps in 1931 was the Boeing Company twin-engine Y1B-9. The ‘Y1’ stood for service test aircraft. It was an all-metal monoplane (single-wing), with retractable wheels. The metal construction of the Y1B-9 produced an aircraft with a significant improvement in strength and durability compared to its wood and fabric biplane predecessors.

The streamlined shape of the Y1B-9 monoplane bomber reduced drag and thereby increased aircraft speed. However, it did retain some old-fashioned design features such as an open cockpit and bombs carried under the wings as it still lacked an internal bomb bay. Only seven were built, remaining in service until 1935.

A game of one-upmanship

The Boeing Y1B-9 series was superseded by the Martin Company-designed and built twin-engine B-10 monoplane bomber, which entered into operational service in 1935. It was also of all-metal construction but unlike the Boeing Y1B-9, it had an internal bomb bay. In addition, it had retractable landing gear, a machine-gun armed turret for self-defence and an enclosed cockpit.

Reflecting the technological leap that the B-10 represented in design and in turn operational capabilities when it rolled off the Martin assembly line, the Air Corps ordered 121 units of the aircraft, the largest order of bombers ever made by the US army’s aviation element up to that point.

Alas, the Martin B-10 series’ time at the forefront of bomber design did not last long. It was replaced by the even more capable Douglas Aircraft Company B-18 twin-engine all-metal monoplane bomber. The B-18 was based on the firm’s twin-engine DC-2 all-metal monoplane commercial transport. In January 1936 the Air Corps ordered 133 units of the B-18, quickly followed by another order for 217 units of a modified version labelled the B-18A.

The future is here

The selection of the twin-engine monoplane Douglas B-18 bomber was not favoured by many in the Air Corps who felt that it was already obsolete compared to Boeing’s new prototype all-metal four-engine monoplane bomber designated the Model 299.

The Boeing 299 was the forerunner of the firm’s B-17 bomber series of Second World War fame. Boeing had designed and built the prototype in response to a 1934 Air Corps requirement for a bomber that could reach faraway American military bases in Hawaii, Alaska and Panama.

The Air Corps selected the twin-engine Douglas B-18 bomber in lieu of the far superior Boeing 299 four-engine bomber prototype for two reasons. First it was less costly, and secondly the Boeing aircraft was destroyed in an accident before the competition was completed. At that time funding for new planes was in short supply and the Air Corps had to stretch its limited budget as far as possible.

Despite its funding shortfalls, the Air Corps was well aware of the operational potential represented by the Boeing 299 four-engine bomber prototype. It therefore went ahead and scraped together sufficient funding to order thirteen upgraded preproduction units of the aircraft for testing in 1935. These pre-production models were designated the Y1B-17.

The first four-engine bomber enters service

Despite being intended solely for testing, the Air Corps went ahead and formed an operational squadron with twelve of its Y1B-17s in 1937. They had been redesignated as the B-17 in 1936 and officially named the ‘Flying Fortress’ in 1938. The sole remaining Y1B-17 was employed by the Air Corps as a test-bed for evaluating different types of power plants for the plane. It was later designated as the B-17A.

The initial production batch of thirty-nine B-17s was designated B-17B with the first examples delivered in June 1939, only three months before the official beginning of the Second World War. However, due to a number of design issues the B-17B was not considered fit for combat employment and all were relegated to noncombat secondary missions in October 1942.

The post-war-designed pursuit planes

The designs of the first two pursuit biplanes acquired by the aviation element of the US army following the First World War had been commissioned during the last year of that conflict. However, neither had been placed into production before the war ended. These were the Orenco Company Model D and the Thomas-Morse MB-3, with fifty units of each aircraft ordered by the War Department.

Unfortunately for both firms, the War Department owned the rights to the designs and they competitively bid further orders, a practice that continued till 1925. Other companies were allowed to underbid them and were awarded the contracts to build the respective aircraft. The building of the Orenco Company Model D went to the Curtiss Aeroplane and Motor Company, and the Thomas-Morse MB-3 to Boeing. Production units of both aircraft were tested with the Air Service in 1919.

Following in the footsteps of the Orenco Company Model D and the Thomas-Morse MB-3 came new biplane fighters. The first post-war-designed model was the Curtiss PW-8, of which the Air Service ordered twenty-five units in 1923. This was followed the next year by the Air Service ordering thirty units of a Boeing pursuit plane designated the PW-9.

Pursuit planes of the 1920s

The Curtiss CW-8 was the starting-point for the company’s famous Hawk series of pursuit biplanes. The name ‘Hawk’ was strictly a company marketing title. The next production model of the Curtiss aircraft taken into service with the aviation element of the US army was the P-6, eighteen of which were ordered in 1928. There were also seventy-one upgraded units, divided across three versions referred to as the P-6A, P-6D and P-6E, which served until 1937.

Fighter plane designation changes

The generic name ‘fighter’ did not come into official use with the aviation element of the US army until it became the United States Air Force (USAF) following the Second World War in 1947. Up to that point, fighters were officially referred to as ‘pursuit planes’. Unofficially, fighters became the accepted term for pursuit planes during the Second World War

Between 1920 and 1924, pursuit planes were designated by the acronym ‘P’ followed by another letter that might describe its engine type or mission. For example, ‘PN’ stood for ‘Pursuit, Night’. In May 1924 the second letters were dropped and only the letter ‘P’ for pursuit plane was used thereafter.

The number/s following the acronym ‘P’ are the acquisition sequence of the aircraft. The gaps between the acquisition sequence of serious production combat aircraft reflect those plane designs that were cancelled.

The Air Service ordered thirteen units of the Boeing P-1 in 1925. This was followed by orders for eighty-three units of progressively improved models, labelled P-1A, P-1B and P-1C. Boeing was also keen on winning additional contracts from the Air Corps for pursuit biplanes. However, their second attempt, a modified version of their PW-8 designated the XP-8 which had a different engine, did not meet the Air Corps’ specifications. The prefix letter ‘X’ stood for experimental.

On their third attempt, Boeing came up with a winning design so impressive in its performance compared to previous pursuit biplanes that both the Air Corps and the US navy ordered it in 1928. The Air Corps’ version was designated the P-12 and the US navy version the F4B-1. Eventually the Air Corps would take into service 365 units of the P-12 series, which encompassed a P-12B through P-12E model ordered in 1931.

Despite their positive attributes, the Boeing P-12 series of pursuit biplanes as well as the Curtiss series had wings made of wood and fabric with open cockpits and fixed landing gear. The only truly modern element in their designs, which had not been seen in the First World War, was their metal fuselages.

All-metal pursuit monoplanes appear

With the advent of the Boeing Y1B-9 twin-engine all-metal bomber in 1931, the Air Corps realized that the next generation of multi-engine all-metal monoplane bombers would be faster than its existing fleet of pursuit biplanes. This pushed the Air Corps to begin considering the acquisition of all-metal pursuit monoplanes.

As the Air Corps lacked the funding required to purchase a new generation of such aircraft, it went to Boeing and asked them to design an all-metal pursuit monoplane that would be less costly than the firm’s existing models. Boeing responded to the Air Corps’ needs and came up with the P-26 series of pursuit monoplanes.

A total of 136 units of the P-26 series was ordered in early 1933, divided between the P-26A and P-26B versions, with delivery beginning later that same year. Curtiss also came up with a pursuit monoplane design at the same time but the Boeing model was judged to be superior.

Despite being all-metal, the P-26A and P-26B still retained a number of First World War design features. These included such things as fixed landing gear, open cockpits and externally-braced wings. Reflecting the higher landing speed that came with a monoplane design, the P-26 series had landing flaps, the latter being a British invention.

Transitional pursuit planes

By the end of the 1930s, it was clear to the Air Corps that the P-26 series was completely outclassed by foreign-designed monoplane fighters. To cope with this growing disparity in operational capabilities between its existing fighter inventory and those of potential foreign opponents, the Air Corps sought out newer-generation monoplane pursuit aircraft.

The Air Corps would replace the P-26 series with two new pursuit planes between 1938 and 1940. These were the almost all-metal Curtiss P-36A (the landing flaps were fabric-covered), and the all-metal Seversky P-35. Both fighters were the first in the Air Corps’ inventory to feature such modern design features as retractable landing gear and enclosed cockpits. There were 178 P-36As built and 77 units of the P-35.

The problem with the P-36A and the P-35 was that they were already obsolete when placed into service. They were both under-armed and lacked self-sealing fuel tanks. The Air Corps was not unaware of these issues and before America’s entry into the Second World War they authorized the design and manufacture of newer pursuit planes superior to the foreign fighters that were then dominating the skies overseas.

Despite its design limitations, the P-36A would remain in service with the Air Corps long enough to see combat during the Japanese attack on Pearl Harbor. Nevertheless, it soon disappeared from front-line service, being replaced by more capable aircraft. The P-35 had already been pulled from front-line service prior to Pearl Harbor.

Ground-attack aircraft

In the early post-war years, the Air Service tried to come up with a dedicated plane for the ground-attack role. Unfortunately, engine technology had not yet developed enough capability to carry the weight of the weapons and armour required of a successful ground-attack aircraft. The Air Service/Air Corps had to make do with modernized units of the First World War era DH-4s in this role until a suitable upgrade could be found.

As a stopgap ground-attack aircraft, the Air Corps took into service in 1927 the first of 143 units of the single-engine Curtiss A-3 biplane, the letter prefix ‘A’ standing for attack. Besides being armed with a number of machine guns, the A-3 also had underwing fittings for carrying bombs. It would survive in service until the early 1930s.

The first dedicated ground-attack aircraft

In 1932 the Air Corps took into service for test purposes thirteen units of a dedicated ground-attack aircraft from Curtiss that was originally designated the YA-8. This new single-engine monoplane had an enclosed cockpit, and wing slots and flaps to assist in reducing landing speed.

Positive feedback from testing the YA-8 led to it being re-designated as the A-8. An improved version was designated the A-12, with forty-six taken into service between 1933 and 1934. It would remain in service until 1941.

At the same time as the order was placed for the Curtiss A-12, the Air Corps ordered five units of a Consolidated Aircraft Corporation single-engine monoplane designated the A-11. Consolidated was formed in 1923. In 1943 it became the Convair Corporation, which also included the Vultee Aircraft Division of the Cord Corporation.

The Consolidated A-11 and the Curtiss A-12 were followed into Air Corps’ service by a ground-attack aircraft designated the A-17. It was designed and built by the Northrop Aircraft Company, formed in 1932. Like the ground-attack aircraft that preceded it, this was a single-engine, all-metal twin-seat monoplane.

The original production run of the A-17 consisted of 110 units. They had enclosed cockpits but fixed landing gear. The follow-on A-17As had retractable landing gear; these were delivered between 1937 and 1938. By the time the A-17A was delivered, Northrop had been acquired by the Douglas Aircraft Company.

In 1938, the Air Corps decided that it wanted only multi-engine attack planes. This meant that the entire A-17 series was surplus to requirements. The majority were therefore passed on to other countries, with those few remaining in the inventory reassigned to non-combat roles.