10/31/2010. Remarks by Johan Visschedijk: "When in early 1940, some seven months before the maiden flight of the first Model 122 (YP-38), Lockheed was granted War Department authorization to sell to France and Britain its Model 322 – a proposed development of the Model 122 to be powered by a pair of Allison V-1710-C15s without turbo-superchargers – the company agreed in exchange to develop and produce at no cost to the US government a prototype of an advanced version of the P-38, formal agreement being signed on April 12, 1940.

With James Gerschler as project engineer, Lockheed drew up preliminary details for its L-121 (a temporary design designation) fighter. Then intended to be powered by two turbocharged Continental IV-1430 liquid-cooled engines, the aircraft was offered in two versions. The single-seat version was to retain the standard P-38 armament – one 0.787 in (20 mm) cannon and four 0.50 in (12.7 mm) machine guns – while in addition the two-seater was to have one 0.50 in (12.7 mm) remote-controlled gun at the end of each tail boom.

During a meeting at Wright Field in May 1940, the decision was taken to proceed with the two-seat version, which received the military designation XP-58. Two months later, the USAAC and Lockheed agreed that with a pair of 1,500 to 1,600 hp Continental engines the two-seater would probably be underpowered and decided to switch to two 1,800 hp Pratt & Whitney XH-2600-9/-11 liquid-cooled engines. Revised specifications for the re-engined XP-58 (Model 20-24) were issued by Lockheed on 10 September, 1940.

Forward-firing armament was increased by a second 0.787 in (20 mm) cannon, and the impractical tail boom guns were to be replaced by a remote-controlled Air Arm dorsal turret housing twin 0.50 in (12.7 mm) machine guns. With these changes, estimated gross weight increased by some 45% to 24,000 lb (10,886 kg) and guaranteed top speed at 25,000 ft (7,620 m) dropped from 450 mph (724 kmh) to 402 mph (647 kmh). Range on internal fuel was still anticipated to be 1,600 mls (2,575 km).

Barely one month after issuing the specifications for the XH-2600 powered aircraft, Lockheed's new project engineer, Neil Harrison, was informed that Pratt & Whitney was suspending its development. Harrison and his team were forced to find new power plants for what was becoming known at Lockheed as the 'X-engine airplane'. Design studies around either a pair of Lycoming XH-2470s, Continental XH-2860s, or Pratt & Whitney R-2800s, were prepared, with Lockheed favoring Pratt & Whitney.

It was calculated that with two of these 1,850 hp engines, the XP-58 would have a loaded weight of 26,000 lb (11,793 kg) and a top speed of 418 mph (673 kmh) at 25,000 ft (7,620 m). The USAAC, however, considered performance to be inadequate and suggested that Lockheed adopt the Wright XR-2160 Tornado engine for the XP-58 (Model 20-59). This 2,350 hp power plant, a 42-cylinder, six-row engine of extremely low frontal area, was highly complex; but, though the R-2160 was problem-fraught from the onset, in March 1941 the USAAC confirmed its support for the Tornado-powered version.

Two months later it issued a Government-funded change order to install cabin pressurization for the pilot and for the aft-facing gunner, and to add a remote-controlled ventral turret. This resulted in the estimated gross weight increasing to 34,242 lb (15,532 kg) – more than twice the weight of the original Continental IV-1430 powered version. Yet, the USAAC insisted on a guaranteed top speed of 450 mph (724 kmh). Estimated range, however, dropped by 300 mls (485 km) to 1,300 mls (2,090 km).

By then the XP-58 was scheduled to be delivered in August 1942 and, to meet this deadline, the project team grew to a peak of 187 by October 1941. Following the United States entry into the war in December of that year, a reassignment of engineering staff to more pressing projects became necessary and the XP-58 design team dropped to a low of twelve people by early 1942.

The reduced-size of the design group, however, had little direct impact on the schedule as the XP-58's intended power plant was lagging much further behind. Nevertheless, at Lockheed's suggestion, the USAAF (renamed from USAAC in June 1941) ordered in May 1942 a second prototype which was to have increased fuel capacity, and other improvements, to more than double the range to 3,000 mls (4,825 km).

Shortly thereafter, thinking within the USAAF began to vacillate and the XP-58 program was thrown into chaos when it was suggested that forward-firing armament be changed to include a 2.95 in (75 mm) cannon, with a 20-round automatic feeder, and two 0.50 in (12.7 mm) machine guns. As this armament was not suitable for an escort fighter, consideration was seriously given to switching the aircraft's intended role to low-level attack.

In turn, this led to studies of other alternative configurations, including a two-seat attack aircraft with six forward-firing 0.787 in (20 mm) cannon, and a three-seat attack/bombing version with a bombardier in the nose, enlarged central nacelle with an internal bomb bay, and with or without the 2.95 in (75 mm) cannon.

As better aircraft, including the Douglas A-26 already in production and the promising Beech XA-38, were available to satisfy the USAAF's low-level/light bombing needs, signals were again switched. In November 1942 it instructed Lockheed to complete the two Tornado-powered XP-58s as bomber destroyers, a mission which by then the USAAF was not likely to have to fulfill. The first aircraft was to have four 1.46 in (37 mm) forward-firing cannon, while the second was to carry a 2.95 in (75 mm) cannon and two 0.50 in (12.7 mm) machine guns.

Both were again to be fitted with dorsal and ventral turrets, and turbo-superchargers-items which had been deleted from the proposed low-level attack version of the XP-58. By then calculated gross weight had soared to 38,275 lb (17,361 kg), top speed had dropped to 414 mph at 25,000 ft (666 kmh at 7,620 m), and range had been reduced to only 1,150 mls (1,850 km).

In January 1943 Lockheed brought some order to this chaos by recommending that only one prototype be built, with interchangeable nose sections to test both types of forward-firing armament. One month later the Wright Tornado engine program collapsed and a final power plant change was submitted by Lockheed. With USAAF concurrence, the sole XP-58 prototype (Model 20-86, serial 41-2670) was to be completed with two turbocharged Allison V-3420-11/-13 twenty-four-cylinder liquid-cooled engines, rated at 2,600 hp for take off and 3,000 hp at 28,000 ft (8,535 m).

With these engines, dummy turrets, and no forward-firing armament, the XP-58 was finally flown by Joe Towle on June 6, 1944, more than four years after its design had begun, from the Lockheed Air Terminal to Muroc AAB. Some twenty-five flights were made by Lockheed personnel prior to the delivery of the XP-58 to Wright Field on October 22, 1944. These manufacturer's trials were marred by turbo-supercharger torching. At Wright Field, the aircraft, for which the USAAF had no longer any need, remained grounded until early 1945 when it was transferred to be used as a non-flying instructional airframe.

Due to the many power plant changes during its protracted gestation period, for which Lockheed could hardly be blamed, and to Air Force mismanagement, the XP-58 program failed to bear fruit. Started at no cost to the Government, it ended costing the US taxpayers $2,345,107 – $451,556 going for the Air Force's requested changes to the first prototype, and the balance covering the Government-ordered, but then cancelled, second prototype.

As completed, the XP-58 had an 800 US gal (3,028 l) internal fuel capacity, which, with auxiliary and drop tanks, could be increased to 1,700 gal (6,435 l). Forward-firing armament, not actually installed on the prototype, would have consisted of either four 1.46 in (37 mm) cannon, or one 2.95 in (75 mm) cannon and two 0.50 in (12.7 mm) machine guns. The dorsal and ventral turrets were to house twin 0.50 in (12.7 mm) machine guns. External loads of 4,000 lb (1,814 kg) could also have been carried."