The first aircraft designed and built by James S. McDonnell’s new company, the Army’s XP-67, is one of the most unusually-shaped aircraft of all time and also one of the least documented. Its combination of pure curvaceous sexiness and a tantalizing (and frustrating) lack of photographic coverage have made it a cult favorite among aviation history buffs for decades. Attempts had been made to tap into historical records at Boeing (merged with McDonnell Douglas in 1997) but the results were meager, to say the least.

We had retired from Boeing in St Louis and were looking for a project. We both love doing research. What better challenge could we have than to take a crack at XP-67? We started by revisiting the Boeing Historian’s office, but nothing new was found there. The National Archives and Records Administration was the next obvious place to look, and in fact there were a number of production drawings of piece parts in their collection. COVID disrupted our plans there, with NARA shut down to visitors for the entire time that we had been given to write a history of XP-67 for Osprey Publishers’ X-Planes series. We had to find other sources, and quickly.

Having made a number of contacts with individuals and organizations, we slowly began to piece together the history of XP-67 that included a variety of photos and drawings that had never been published. This research kept expanding in scope, bringing in all kinds of factors that we hadn’t been aware of. Along the way we uncovered many things that surprised us, given that we had previously only known the top-level story that has been repeated so many times over the years. Here are our top 5, and they may surprise you, too:

1. XP-67 actually did have a nickname! “Moonbat” wasn’t used at the time, but it’s completely understandable why it’s stuck informally for so long. The airplane does look bat-like, thanks to its use of extreme blending of the engine nacelles into the wing and the wing into the fuselage. In conventional aircraft the blending is much more restrained, and is called “filleting.” So, XP-67’s most distinctive external feature is what led Army test pilots to give it a nickname in their official report: “The XP-67 is known as the ‘Flying Fillet’; any longitudinal cross section through the airplane being an airfoil section.”

2. XP-67’s engines weren’t particularly troublesome! And they definitely weren’t prone to catching fire. Contemporary records don’t always make a clear distinction between overheating and actual fire, but in all but one case those events were due to McDonnell Aircraft Corporation (MAC) designed peripherals such as ducts and valves rather than anything failing in the engines. Literally the only time that an engine component failed and started a fire was during XP-67’s last flight, after 8 months of flight testing had been completed. It’s worth noting that the only other aircraft ever to fly with the same Continental I-1430 engines was Lockheed’s XP-49, and no fires or serious mechanical failures occurred there either. It’s been said many times that they failed to deliver their full rated horsepower, which led to sluggish takeoff and high-speedperformance. But in fact, an Inter-Office Memorandum dated 19 January 1944, so early in the program that flight testing had barely begun, the Army program coordinator for XP-67 to the Chief of Aircraft Projects at Wright Field noted that as a result of extensive engine testing in the full-scale nacelle fixture “it is the opinion of this office that the engine has performed satisfactorily. This opinion is borne out by tunnel tests of the full-scale nacelle at Wright Field, during which engine difficulties were practically non-existant [sic] and the engine delivered its rated 1600 hp for protracted periods of time.” Later in the program, as rumors of fires apparently started to spread, the Chief of Technical Staff at Wright Field noted that apart from a fire on the first flight (resulting from failure of a duct carrying exhaust gases, nothing to do with the engine itself) – “The fifth flight occurred on 25 March 1944. Approximately fifty flights have been accomplished and no serious functional difficulties have been encountered.”

3. Army pilots only flew XP-67 a handful of times! Virtually all of the XP-67’s test flying was done by MAC Chief Test Pilot Ed Elliott. Three Army pilots flew a total of just five flights and a total of around 4 hours split between them, hardly enough time to become familiar with the aircraft, much less make more than superficial judgements about its performance. Some of their critiques are baffling, such as comparing the large twin-engine XP-67’s maneuverability and turn radius with that of a small single-engine P-51B and naturally finding that the latter was superior, a complete irrelevancy since XP-67 was meant to destroy bombers with its six 37mm cannon (never actually fitted) rather than dogfight with enemy fighters. There are indications in the surviving program correspondence that other Army pilots may have made single flights at times, but they appear to have been opportunistic rather than part of any organized test activities. 

4. XP-67’s rollout and final demise happened very close to each other! McDonnell’s facilities at Lambert Airport in St Louis were at the north side of the field, and that’s where XP-67 was rolled out in November 1943. On a windy day in September 1944, it made its last flight, landing on fire and being abandoned by its pilot on a taxiway that was on the then-southwest corner of the airport, less than 1,000 yards from where it had been rolled out. We got permission to go inside the perimeter of today’s St Louis Lambert International Airport and stood on both sites, which were easily visible from each other.

5. XP-67 wasn’t the only “X-fighter” at Lambert Airport in 1943-44! McDonnell’s facilities were directly across the runway from the huge new plant that Curtiss-Wright had constructed for building mostly variants of the SB2C Helldiver family. At the same time that XP-67 was being developed, Curtiss-Wright was building their XP-55Ascender, which they rolled out in July 1943, just 4 months before XP-67 made its first appearance, and was lost in November 1943 while flying from Lambert, just two weeks before XP-67 left the assembly building. The second XP-55 saw daylight in January 1944, while XP-67 was in the process of making its first four flights from Scott Field in Illinois. But the XP-55 number 3 emerged in April 1944, while XP-67 was actively test flying from Lambert. It’s very likely that both aircraft were within sight of each other at one or more times, but sadly no photos have been found to show this. This near-neighboring of two Army WW2 fighter X-planes is unique!

Those were some of the most interesting things that we discovered while researching and writing our book. But on a personal note, the one that doesn’t get a mention is the fact that much of XP-67’s test flying and XP-55’s too, including (we believe) XP-67’s catastrophic final flight, took place in the airspace directly over our house! At the time it hadn’t yet been built, of course, but it’s still something that we like to imagine when we look up at the sky.

– Steven Richardson was an aero engineer who worked at McDonnell and Boeing, and Peggy Mason was in comms at Boeing, You can order their McDonnell XP-67 “Moonbat” (X-Planes) book here. Review here.

The Hush-Kit Book of Warplanes can be ordered here, Vol 2 of The Hush-Kit Book of Warplanes can be supported here