History
Background
With the Curtiss P-40 Warhawk entering production by 1940, Curtiss began an ambitious effort to develop a superior aircraft that could replace the P-40. The US Army Air Corps (USAAC) was interested in supporting these efforts and awarded a contract to fund the development of potential successors.
Curtiss initially designed the XP-46 prototype, which shared similarities with the P-40, except it was built to be smaller and equipped with an inward-retracting undercarriage. The XP-46 took its first flight on February 15, 1941, but it offered no meaningful performance improvements over the P-40 and was outright rejected by the USAAC.
After the XP-46 did not secure production orders, Curtiss shifted focus to a new design: the Model 88, later designated the XP-53 by the Army. This model was intended to use a Continental XIV-1430-3 engine. However, engine development issues led to the cancellation of the XP-53 before it could be built.
Curtiss then moved on to the development of Model 90, which was designated XP-60. At the Army’s request, the XP-60 was initially designed to be powered by a Rolls-Royce Merlin engine and include laminar-flow wings, a type of airfoil designed to reduce drag and make the aircraft fly faster. Curtiss produced four airframes during this project, which resulted in nine different designations: XP-60, XP-60A, YP-60A, P-60A, XP-60B, XP-60C, XP-60D, XP-60E, and YP-60E. These versions represented Curtiss’ long and desperate attempts to develop a worthy successor to the P-40—efforts that lasted until late 1944.
XP-60 - Model 90/90A - Initial Prototype
The Model 90, also known as the XP-60, was Curtiss’ initial prototype in the P-60 series. It was ordered on October 1, 1940, specifying one of the two XP-53 prototypes to be reused and developed as the XP-60. Two months later, the Army requested the incorporation of a Rolls-Royce Merlin engine, leading to the aircraft’s redesignation as the XP-60. This prototype used the P-40D’s fuselage and tail assembly combined with the laminar-flow wing. It retained the inward-retracting undercarriage from the P-40.
Powering the XP-60 was a 1,300-hp Packard V-1650-1 Merlin engine, a license-built, liquid-cooled engine fitted with a single-stage, two-speed mechanical supercharger. The radiator was relocated farther back under the nose to accommodate this configuration. Planned armament included eight M2 Browning machine guns, each with 250 rounds, for a total of 2,000 rounds. To enhance combat survivability, the aircraft was designed to include armor protection for the pilot and self-sealing fuel tanks.
The preliminary design for the XP-60 was presented to the Army on January 16, 1941, and subsequently approved by the Army. Construction began shortly after, and by September 16, 1941, the aircraft initiated taxi trials with the Merlin engine installed. Just two days later, it made its maiden flight, only 11 days after the first flight of the XP-46. During initial testing, undercarriage failures caused damage, prompting design tweaks like enlarging the vertical tail. These changes led to a redesignation of the Curtiss Model 90A. Self-sealing fuel tanks and a bulletproof windscreen were later installed. By the spring and summer of 1942, the aircraft was sent to the Army Air Forces for further testing.
Despite modifications being made prior to the subsequent testing, the XP-60’s engine underperformed as it failed to deliver the expected power output. Additionally, the aircraft’s wings did not meet manufacturing standards, meaning they failed to provide the full benefits of laminar flow and negatively impacted its flight performance. The XP-60 only achieved a top speed of 387 mph.
With the aircraft falling short of Army expectations, with concerns about a potential shortage of Packard-built Merlin engines, the Army instructed Curtiss to experiment with installing alternative engines in the XP-60 airframe. This led to the development of the XP-60D in August 1942, which was equipped with an uprated V-1650-3 Merlin engine, the same engine used in the P-51 Razorback Mustang. The conversion to the P-60A and P-60D marked the end of the first phase of XP-60 development.
