Throughout history, numerous groundbreaking aircraft prototypes have served as catalysts for the advancement of new technologies. These innovations range from revolutionary engine designs and aerodynamic concepts to unconventional airframe configurations. X-planes spanning various eras have consistently showcased the forefront of engineering prowess. In the early 1940s, McDonnell amalgamated a plethora of novel ideas into its XP-67, aiming to surpass the performance of existing interceptors.
While a prudent approach to introducing novel aircraft designs typically involves integrating a limited number of untested concepts at a time, McDonnell took a different route with the XP-67, dubbed the “Moonbat.” Engineers were granted unprecedented freedom to reimagine every aspect of the airframe and powerplant, resulting in a comprehensive integration of innovative ideas into a singular aircraft.
At first glance, the XP-67 exudes the characteristics of an aerodynamic masterpiece. Featuring a semi-blended-wing body, a laminar-flow airfoil, and seamlessly rounded junctions, the airframe boasts a unified, organic aesthetic. This meticulous sculpting was driven by McDonnell’s ambitious target of achieving a blistering top speed of 472 mph, a figure crucial for securing a lucrative military production contract.
Beneath its unconventional aerodynamic exterior, the XP-67 harbored additional complexities, particularly in its powerplant. Equipped with the new and untested Continental XL-1430 inverted V-12 engines, each boasting 1,350 hp and liquid-cooling, these powerplants held significant promise upon their development in the 1930s.
They provided approximately 1 horsepower per cubic inch of displacement, surpassing the Rolls-Royce Merlins in power while maintaining a smaller size. However, this advantage came with a drawback: a less favorable power-to-weight ratio. As the XP-67 aircraft underwent development, modifications to the airframe compounded its weight, escalating from the initial target of 18,600 pounds to over 25,000 pounds.
With increased weight came aerodynamic challenges. While the aerodynamically optimized airframe enhanced efficiency at high speeds, it posed significant issues elsewhere. Despite innovative leading-edge intakes designed to cool the engines, overheating and occasional fires occurred during initial ground tests and taxi runs. This was likely due to the extremely streamlined cowlings and ductwork, prioritizing low drag over adequate cooling. The first flight in January 1944 had to be abbreviated due to in-flight overheating.
Engineers swiftly revised the engine cooling system to mitigate these problems. However, addressing the aerodynamic and handling issues proved more challenging. Subsequent test flights revealed instability concerns, casting doubts on the aircraft’s ability to recover from spins. Furthermore, climb performance fell short of expectations, approach speeds increased from 76 to 93 mph, and the aircraft’s maximum speed peaked at 405 mph—67 mph below the intended target.
To address these challenges, the design team investigated alternative engine and propeller options. Despite the potential shown by the unproven Continental, which had been competitive in the previous decade, advancements in the Rolls-Royce Merlin engine had surpassed its capabilities, and the emergence of jet engines was becoming increasingly prominent. The XP-67, plagued by numerous issues, seemed to be facing an inevitable end.
In September 1944, that end arrived. During a test flight over St. Louis, one of the engines caught fire. Despite the pilot’s successful emergency landing at Lambert Field (KSTL), the flames engulfed the aircraft before emergency crews could extinguish them, resulting in its destruction. With mounting difficulties and a second prototype still far from completion, the program was terminated, and both prototypes were scrapped.
