Prototype aircraft of the World War II era, whether or not they pointed the way to a long production run, were sometimes discarded once their primary flight test work was concluded.
But large multi-engine aircraft were hard to come by, and the U.S. Navy and Army Air Forces coveted outsized airframes for secondary purposes. A number of one-off large aircraft dodged the scrapper throughout World War II.
If it seems physically and fiscally impossible today to keep a single copy of an aircraft flyable and operationally serviceable, such was not the case during the war. Prototypes were constructed with techniques and materials that were readily available, so repair or replacement of airframe parts was up to the skills of the crews who tended the aircraft.
Let’s look at four bomber prototypes and two monstrous transport designs, all six of which earned their keep for years as the only example built.
XPB2Y Coronado Prototype
Consolidated Aircraft had already earned a solid reputation for building flying boats when it launched the XPB2Y Coronado, designed as a seaworthy patrol bomber for the U.S. Navy, in 1937.
But the prototype’s original single vertical fin and rudder proved incapable of keeping the first Coronado out of flying difficulties, including dramatic spins that required full power for recovery.
The Consolidated team went to work, adding two auxiliary vertical fins to the original offering before radically redesigning the set of tail surfaces with two vertical fin-and-rudder assemblies on a horizontal tail with marked dihedral. The new tail fixed the flight issues, but major changes to the fuselage and an enlarging of the vertical fins and rudders, making them something akin to the tail of a B-24 Liberator, made the first production Coronados substantially different from the experimental prototype.
One might presume the Navy would cast off the one-of-a-kind XPB2Y-1 prototype once it was surpassed in production, but the prototype’s painful development made it a literal million-dollar airplane. Now viable and flyable, the prototype Coronado became an unarmed transport for admirals in the Pacific, logging thousands of air miles while serving throughout World War II.
Nicknamed the Blue Goose, the prototype Coronado was scrapped quietly in San Diego in August 1945.
XB-15
Boeing built the giant XB-15 to meet an Air Corps proposal for a bomber with a range of 5,000 miles. It first flew in October 1937. The XB-15 proved it had range and load-carrying ability, but ultimately lacked the speed and altitude performance required of a successful bomber. Its listed service ceiling of 18,900 feet was easily bested by smaller four-engine production bombers that followed.
Much as the Navy chose not to scrap the prototype Coronado, the Air Corps assigned the lone XB-15 to the 2nd Bomb Group at Langley Field, Virginia. If it was less than optimal as a bomber, the broad-winged XB-15 could carry tonnage, as it proved on Feb. 4, 1939, by airlifting more than one-and-a-half tons of relief supplies to Chile following an earthquake. That July, the XB-15 carried a record payload well over 15 tons to an altitude above 6,500 feet, shattering previous records.
By 1943, the XB-15 was modified to enhance its use as a large transport, and redesignated XC-105, with the “C” representing cargo. Albrook Field, Panama, became home base as the XC-105 supported cargo demands in the Caribbean region.
The Army Air Forces put the XC-105, called “Grandpappy,” out to pasture in December 1944. In May 1945, the order was issued to dismantle the XC-105 in Panama.
Seven years of flying service attest to the ability of maintainers to keep the one-off XB-15/XC-105 airworthy.
XB-19
Douglas began work on its mammoth XB-19 bomber in 1935, but gestation was complex and the XB-19 did not fly until June 27, 1941.
In that six-year span, the advancing state-of-the-art in both bombers and fighters showed the XB-19 would not be viable as a bomber. But it was a fabulous test bed and technology demonstrator, heralded as the largest aircraft in the world for a time. And it was huge, with a wingspan of 212 feet. By comparison, a B-52 spans only 185 feet.
The flying XB-19 enhanced confidence in other proposed very large airframe designs to follow. The experimental Douglas bomber’s contributions to aviation culminated in flights for the Air Force’s All Weather Flying Service, a laboratory research environment, in the early post-World War II period.
In 1946, the XB-19 entered storage in Arizona, followed by scrapping in 1949.
XPB2M Mars
The last two Martin JRM Mars seaplanes entered museum retirement this year. They owe their successful, and long, careers to the prototype Mars, built as the XPB2M-1 patrol bomber.
The Martin company, like Consolidated, had a long and solid history of building flying boats. The XPB2M Mars built upon Martin’s experience with the successful, and smaller, PBM Mariner.
Delays, including an onboard fire, pushed back delivery and tests of the XPB2M-1 patrol bomber seaplane into 1942. As the war progressed, the Navy de-emphasized large flying boat bombers for the patrol mission, and negotiated successfully to get a piece of B-24 Liberator long-range land-based bomber production earmarked for the Navy.
The original twin-tail prototype Mars patrol bomber was modified to become a large (200-foot wingspan) transport as the XPB2M-1R, flying cargo and passengers for much of World War II.
This prototype validated an upgraded design of the Mars as a purpose-built transport with a small, but valuable, production run as the single-tail JRM.
The prototype retired from Navy service at Alameda, California, in March 1945. But the prototype Mars had more rounds of service, starting the following month when it returned to Martin in Maryland to stand in as a crew trainer for the impending JRM transport versions.
A final stint as a maintenance trainer kept the prototype around until 1949 when the scrapper claimed it.
XC-99
This visit to the land of one-off giants ends with an aircraft that survived its flying life — sort of.
The sole Convair XC-99 is a purpose-built transport based on the intercontinental B-36 bomber the company began designing during World War II. Just as the B-36’s ultimate layout changed, so did the XC-99 evolve on the drawing boards.
What emerged from the Convair home plant in San Diego in 1947 was a blunt double-deck fuselage essentially mated to B-36 wings and tail.
While only one C-99 was built, it shared a number of items in common with the fleet of B-36s, enhancing its maintainability. Convair pitched an evolved production version to the Air Force, and also envisioned an airliner based on the C-99.
Production models would have featured pressurization and this led Convair to incorporate more aluminum and less magnesium into the construction of the XC-99 than in the B-36, since aluminum was expected to have a better service life over many pressurization cycles.
Instead of investing in a fleet of huge C-99s, the Air Force bought more modest four-engine transports like the C-124 Globemaster and C-97 Stratofreighter.
The Air Force employed the XC-99 to carry large tonnage. Sometimes it was a load of needed engines, sometimes the support equipment for a deploying fighter group. The trips, often setting unofficial weightlifting records, lasted about a decade.
By the summer of 1957, evidence of structural fatigue plagued the XC-99 after logging 7,430 flight hours and carrying an astounding 60 million pounds of cargo.
The XC-99’s permanent grounding in August 1957 set the stage for decades of open-air display adjacent to Kelly Air Force Base in Texas.
Finally, between 2004 and 2008 the XC-99 was disassembled and shipped in sections to the National Museum of the U.S. Air Force at Wright-Patterson Air Force Base in Dayton, Ohio.
The careful job of taking the huge airframe apart was conducted by Ben Nattrass and his Worldwide Aircraft Recovery team.
As the museum’s restoration team surveyed the XC-99’s components, the degree of corrosion and the amount of major reconstruction of structural parts was deemed too serious for the museum’s resources to cope with in a predictable amount of time.
The parts of the XC-99 traveled once more to the museum’s holding area at Davis-Monthan Air Force Base in Arizona, pending a restoration solution in the future.
Postscript
In researching this article, the work of two authors stands out for their meticulous chronicling.
Dennis R. Jenkins wrote the seminal history of the B-36, “Magnesium Overcast,” (Specialty Press, 2001), that covers details of the XC-99 venture. Capt. Richard Hoffman’s book, “Consolidated PB2Y Coronado” (Ginter Books, 2009), includes details about the development and demise of the prototype Coronado.
And a note in passing: Ben Nattrass of Worldwide Aircraft Recovery, the creative mind who figured out how to carefully dismantle and move everything from SR-71s to the XC-99, died in April 2023.
In addition to the many historic aircraft Ben and his team moved to museums around the country, his legacy includes the legion of friends he made in the process.
