A shortage of modern monoplane fighters worldwide preceding World War 2 (1939--1945) meant that virtually any and all fighter designs - whether inherently successful or not - were entertained by global powers for frontline service. The P-66 / Model 48 "Vanguard" was a development appearing just at the outbreak of the war in Europe and was designed by Richard W. Palmer for the Vultee Aircraft company as a dedicated fighter. It featured the usual modern traits of the time - an enclosed cockpit, streamlined metal fuselage, low-set monoplane wings and a retractable undercarriage - but was only produced in 146 total examples. Originally ordered by Sweden, the United States instead retained control over the Vanguard stock and offered it to allies in Britain and China, the latter becoming the primary operator of the design for its time aloft.
Palmer headed the design of four similar aircraft to accomplish different dedicated over-battlefield tasks - V-48, BC-51, B-54 and BC-54D. These were used to fulfill fighter, basic training, advance training and basic training requirements respectively. The V-48, or Model 48, in particular, was to become a single-seat, single-engine fighter utilizing the latest in metal-skinning technology. Power would come from a Pratt & Whitney R-1830-S4C4-G 14-cylinder engine of 1,200 horsepower and driving a three-bladed propeller unit at the nose. The cockpit was set at midships and sported a framed canopy and raised fuselage spine. Wings were low-mounted monoplane assemblies fitted forwards and the tail of a conventional single-rudder design. The undercarriage was of a typical "tail dragger" configuration consistent for the times but retractable for drag reduction.
The Model 48 achieved a first flight in prototype form on September 8th, 1939. Germany had invaded neighboring Poland back on September 1st and world powers - including the United States an ocean away - took notice of the evolving situation in Europe. Flight testing revealed several deficiencies in the design of Model 48, including engine cooling issues and stability problems, which forced a modification of the tail unit surfaces and internal engine intake arrangement. However these changes did little to alleviate the ongoing issues. Model 48 collided with a Lockheed Sirius utility aircraft on May 9th, 1940 which caused considerable damage to the Model 48's undercarriage (one leg was wholly lost). After a safe landing the aircraft was rebuilt with changes enacted but this also served to delay the Vanguard program as a whole.
A second prototype followed as Model 48X (V-48X) and this one flown for the first time on February 11th, 1940 with a new engine cowling design and revised wing structure. Intended armament for the finalized production forms was to be 4 x 0.30 caliber Browning medium machine guns and 2 x 0.50 caliber Browning heavy machine guns for a considerable forward "punch". No provision was made for bomb-carrying.
With the worsening situation in Europe, the Swedish government moved to secure the modern fighter from Vultee - 144 being contracted for. These were designated Model 49C (V-49C) and prepped for serial production - to include the full machine gun armament and more advanced R-1830 radial engine for better high-altitude performance. A production airframe flew for the first time on September 6th, 1941.
The United States government blocked deliveries of the Vanguard to Sweden and elected to keep 50 of the aircraft as "P-66" trainers for the United States Army Air Forces (USAAF). The remainder of the stock - which had completed production in April of 1942 - was briefly evaluated for Royal Air Force service which planned on 100 of the aircraft to serve as advanced trainers in Canada under the "Vanguard I" name. Unimpressed with the design, the aircraft was passed on to serve on the Chinese Front against Japan. Originally, the American Volunteer Group (AVG) were to rely on the model but this later changed to Chinese Air Force ownership. At least 104 were earmarked for the journey (by way of India) but it appears that as few as 79 were actually made available. These fought on under the Chinese Air Force flag from 1942 onward to which their wartime service proved rather forgettable - the American fighter largely outmatched by the Japanese offerings of the period and many destroyed when strafed on the ground. The Curtiss P-40 "Warhawk" fighter (detailed elsewhere on this site) supplemented Chinese losses. Chinese wartime P-66 squadrons were the 3rd and 11th Pursuit Groups.
American P-66 use ended during 1943 while Chinese airframes were still available to an extent heading into 1947.
As completed, the P-66 exhibited a maximum speed of 340 miles per hour, a cruise speed of 290 miles per hour and a service ceiling up to 28,200 feet. Rate-of-climb was 2,520 feet-per-minute.