The McDonnell P-62/P-73 was one of the the most unconventional American aircraft designs of World War 2, and despite a difficult development, would end up as one of the highest performance designs of the war, and would serve well into the 1950s.
One of the first designs produced by McDonnell, the Nightstar was developed in response to the same request for a radar equipped night fighter as Northrop's P-61 Black Widow. Due to the effectiveness of German night bombing efforts against the UK (author note: seen as more of a threat than OTL), development of the night fighters was a high priority. Both the P-61 and P-62 prototypes first flew in late 1941; both were radar equipped designs armed with a combination of 20mm cannons and .50 caliber machine guns, and three crewmen. However, the similarities ended there. The Northrop design used a somewhat conventional twin-boom design, with two R-2800 engines and the three crewmen in a central pod. The P-62 instead used a pair of Allison V-1710 engines driving a pair of pusher contra-rotating propellers. These propellers were equipped with explosive bolts to jettison them in the event the crew was forced to bail out, another previously unknown feature.
Most unusually, it had a canard design, with the slightly swept wings mounted far back on the fuselage and pitch control provided by surfaces as the nose. McDonnell's intent with this design was to reduce the frontal area and minimize drag; while the P-62 did have a pair of wing-mounted pods to house the landing gear and turbochargers for the V-1710s, they were much smaller than the twin-booms on the P-61. Each pod also had a pair of .50 caliber machine guns in the rear, aimed by the gunner using a periscope system.
McDonnell had originally intended for the design to use a pair of Allison V-3420 engines (essentially a pair of V-1710s), but the protracted development of this engine led to the use of the V-1710, leaving the prototype P-62 underpowered. As a result, despite its lower drag design, the P-62 demonstrated only marginally better performance than the P-61 in testing. In addition, the engines suffered overheating issues, and the periscope aiming system was clumsy compared to the more conventional turret system on the P-61. The USAAF opted to procure the P-61, and only a half dozen YP-62s were built for development and testing purposes. However, the expected performance of the Nightstar if the V-3420 panned out, combined with its innovative design, led the USAAF and McDonnell to continue development. By the time the first YP-62 first flew, the P-73 was already under development.
Although the P-73 looked outwardly similar to the P-62, internally it was a very different aircraft. By late 1942 the issues with the V-3420 were mostly worked out, and it was ready for installation in the P-73, although the design would compete with the B-39 for the valuable engines. (Author note: another PoD. Also, dual installation of V-3420s was actually considered in OTL;
https://enginehistory.org/Piston/Allison/AllisonImages/AllisonImages.shtml) The gunner was eliminated, reducing the crew to two, and both were now equipped with early ejection seats. The .50 caliber machine guns were moved to the front of the pods (and increased in number to six total). The 20mm cannons were also relocated to a ventral pod, rather than the cheek mounts on the YP-62.
With over 5,000 horsepower installed, the P-73 had incredible performance for such a large aircraft, being capable of over 400 mph in level flight. It did have some flaws, such as a high wing loading and inferior maneuverability compared to the P-61, though it did have a superior roll rate. Despite this, the USAAF immediately ordered the P-73 into production. After a small batch of P-73As, the first major variant was the P-73B, which made its first deployments to the UK in late 1943.
The arrival of the aircraft was a nasty surprise to German bomber crews, even those flying high performance aircraft like the He-177 and even the Ju-288. The He-177 was easily run down by the P-73 (when it was not setting itself on fire); only the limitations of the early radars on the American aircraft and the low numbers of P-73s present gave them a chance at survival. The Ju-288 was a more difficult opponent, being nearly as fast as the P-73 and heavily armed. Still, the P-73 (combined with British night fighters like the Mosquito) took a grievous toll on German night bombers in 1943 and early 1944, leading to the cessation of the pinprick bombing campaign. One notable variant of the P-73 deployed in 1944 was the P-73C, which mounted four .50 caliber machine guns in the upper fuselage in an installation similar to many German night fighters. While effective, the increased drag and removal of a fuel tank to support the installation made it less popular than the P-73B.
Aside from the P-73B, the main variant produced during World War 2 was the P-73F. A day fighter version, the P-73F had uprated engines, a bubble canopy, and a cleaner design with no ventral cannon pod. This allowed it to reach a top spead close to 450 mph, and with an armament of three 37mm cannons (in the nose replacing the radar) and six .50 caliber machine guns, it was also incredibly heavily armed. The P-73F saw use as an escort fighter in the Pacific and also limited use in Europe, where by 1945 its cannons were used more for strafing than for engaging the heavily depleted Luftwaffe. It would likely have seen heavy use in the invasion of Japan, had those operations gone ahead.
Unlike most World War 2 designs, production of the P-73 continued for a brief period after the end of the war. Roughly 400 P-73Ks were produced, to fill the need of both a night fighter and a long range escort fighter pending the development of jet aircraft with superior performance. The P-73K had the definitive version of the V-3420, and with other tweaks to its design, could (barely) reach 500 mph at high altitude. While this was slower than even early jet fighters like the F-80, the P-73K (redesignated F-73K in 1947) had a longer range and could operate in any weather. Several squadrons were still in service by the time of the Korean War, where the F-73K served admirably as a night fighter and ground attack aircraft. Despite this, in the early 1950s, the F-73 was rapidly replaced by superior jet powered aircraft. However, several Air National Guard squadrons continued to use the aircaft until the late 1950s. Interestingly, one of the last F-73K squadrons on active duty was selected in 1955 for tests with the US Navy Sparrow I radar guided missile. Though it only used the missile briefly, the F-73K gained distinction was one of the first missile-armed interceptors.
