<p>The two-seat light observation aircraft had been an essential adjunct to US Army operations, a concept dating back to World War I when, initially, this was about the only task that an aircraft was considered capable of carrying out effectively. In the years between then and the late 1930s, observation aircraft had, of course, been developed to offer much improved performance, some with high-lift devices which made it possible for them to operate into and out of small unprepared areas.
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<p>When, in 1940, the US Army Air Corps realized the need to reinforce its aircraft in this category, specifications were circulated and these resulted in contracts for three examples each of Bellanca and Ryan designs, designated YO-50 and YO-51 respectively. Stinson, however, were awarded a contract for 142 of its design under the designation O-49. Built by Vultee after its acquisition of Stinson during the summer of 1940, the O-49 appeared as a braced high-wing monoplane, with an all-metal basic structure, part metal- and part fabric-covered. To provide the essential low-speed and high-lift performance, the whole of the wing leading edge was provided with automatically operated slats, and the entire trailing edge was occupied by wide-span (almost two-thirds) slotted flaps and large slotted ailerons which drooped 20 degrees when the flaps were fully down. The non-retractable tailwheel-type landing gear was designed specially for operation from unprepared strips. The powerplant consisted of a 285-hp (213-kW) Lycoming R-680-9 radial engine with a two-bladed constant-speed propeller. An enclosed cabin seated two in tandem, and the pilot and observer had excellent vision all around, above and below.
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<p>A second contract covered the construction of 182 O-49As, which differed by having a slightly longer fuselage, minor equipment changes and detail refinements. Designation changes in 1942 resulted in the O-49 and O-49A becoming the L-1 (liaison) and L-1A, respectively. Both versions were supplied to the RAF under Lend-Lease and these were given the British name Vigilant. The designation O-49B (later L-1B) was applied to three O-49s converted to serve in an ambulance role: these could accommodate one stretcher and had a special loading door in the upper surface of the fuselage. Other designations included an L-1C ambulance converted to have a different internal arrangement; 21 L-1Ds modified from L-1As to provide pilot training in glider pick-up techniques; seven L-1s equipped for the ambulance role and provided with amphibious floats as L-1Es; and five similarly-equipped L-1As redesignated L-1Fs.
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<p>No further production of new Vigilant aircraft followed, for the type was superseded by the more effective lightweight Grasshopper family. Nevertheless, Vigilants saw wide use in both the European and Pacific theaters, the RAF operating many of its aircraft for artillery liaison in Italy, Sicily and Tunisia.
<item><hi format=bold>Performance:</hi> maximum speed 122 mph (196 km/h); service ceiling 12,800 ft (3,900 m); range 280 miles (451 km)
<item><hi format=bold>Weights:</hi> empty 2,670 lb (1,211 kg); maximum take-off 3,400 lb (1,542 kg)
<item><hi format=bold>Dimensions:</hi> span 50 ft 11 in (15.52 m); length 34 ft 3 in (10.44 m); height 10 ft 2 in (3.10 m); wing area 329 sq ft (30.56 m<su>2</su>)
