The Martin XB-48; Neither Simple nor Innovative Enough

June 7, 2023

The impact that Glenn Martin had on aviation is difficult to overstate. On top of being one of the first record breaking pioneers, starting his career as a designer and builder just shortly after the Wright brothers first flew, Martin also had a great influence on some of the biggest names in American aviation.

Donald Douglas, James McDonnell and Lawrence Bell all worked for Martin at stages of their careers before going on to form their own aviation companies. Martin even taught one William Boeing how to fly and sold him his first aircraft.

This ability to recruit and inspire talent meant that the Martin Company was a world leader in aviation for several decades, particularly in the field that was getting more and more attention in military circles – bombers. In fact the Martin company built the United States first indigenous purpose-made bomber, the MB-1, in 1918.

In 1930, Martin began development of the B-10, a bomber that essentially set the standard for the aircraft that would go to war a decade later. Indeed, throughout the Second World War, Martin was one of the major producers of bombers for the United States and its allies, building thousands of Maryland, Baltimore and B-26 aircraft that between them fought in every theatre of the war. In fact, it was two aircraft built at Martin’s Nebraska factory that arguably had the biggest impact on warfare in history; the Enola Gay and the Bockscar, which respectively dropped the atomic bombs on Hiroshima and Nagasaki.

But the end of the war also meant new challenges for Martin as the vast improvements in aviation technology meant that, if they wished to retain their position as one of the United States most innovative and preeminent bomber builders they had to adapt to the times. And somewhat strangely their offering, the XB-48, really blew it.

I guess you can only be right so many times before getting it wrong.

The XB-48 genesis lay in a USAAF requirement that was issued in 1944. The Air Force was interested in the potential that the new jet engines under advanced development at the time offered for the future of their bomber force. And because of that whole war thing, making cost largely irrelevant, and the perceived critical need for future jet bombers, the USAAF basically told North American, Convair, Boeing and Martin to get on and create aircraft to meet their requirement as quickly as possible.

The specification stated that the new aircraft needed to be capable of a range of 3,000 miles (4,828km), a tactical operating altitude of 40,000 feet (12,192m) and a maximum speed of 550 mph (885km/h).

The end of the war in 1945 didn’t take the pressure off, and though many military contracts were cancelled, the deteriorating relationship with the Soviet Union meant the USAAF were even more determined to get jet bombers into service quickly. A review of the four competitors in 1946 revealed that both the North American XB-45 and Convair XB-46 were closest to being ready for production, while Boeing’s XB-47 and Martin’s XB-48 needed several years of further development.

Indeed, the North American offering, while being on paper the least capable of the designs, was the closest to the existing piston-engine bombers in service and so was ordered as the B-45 Tornado to serve as an intermediate type to get a jet bomber into service. This spelt the end of the Convair XB-46, but it was also recognised that the two remaining competitors had great potential and so it was announced that any further decision would be delayed until both aircraft had flown and proper assessments able to be conducted as to which was superior.

In December 1946 a contract was signed that would see Martin build two XB-48 prototypes for trial examination and possible selection.

The aircraft very much was a combination of the old with the new.

The XB-48 combined Martin’s wartime experience of building a sleek tubular fuselage combined with an extremely thin, straight wing. The three-man crew was to be formed of the pilot and co-pilot sitting in tandem under the glazed canopy, while the navigator/bombardier occupied a glazed nose position at the very front of the aircraft. Armament was projected as twin remote-controlled and radar directed 0.50-calibre machine guns in the tail for defensive purposes, plus a maximum bombload of 22,000lb (10,000kgs).

The bomb bay was extremely large as the specification called for the aircraft to both be capable of carrying the massive T-14 bomb, the American version of the British Grand Slam bomb and the early, and extremely bulky, nuclear weapons.

This was all fairly standard for the aircraft of the day, and the innovations came largely with the landing gear and propulsion.

The wing was so thin that it was impossible to use a conventional landing gear and so Martin had to utilise something new – a tandem undercarriage that used outriggers in the wings to stabilise the aircraft on the ground.

Martin had in fact experimented with this concept on the XB-26H and concluded that it was the optimal design for the XB-48.

The same cannot be said about the engine layout. To be fair to Martin, jet engines were in the very first stages of utilisation, and so Martin sought to fit six General Electric J35 turbojets that produced 3,820lbf each. And they did so in a unique configuration, with three engines all effectively housed in a single rectangular pod under each wing.

These had the top of the nacelles fared into the wing for better streamlining and to provide additional lift, and the gaps between the engines within their pods was taken up by air ducts for cooling.

But the design was to prove very much a learning experience. The triple engine pods proved extremely inefficient, and test pilots reported being able to literally see the air flow stacking up in front of the engines during test flights because of the way the engines were placed together.

This obviously impacted performance and engine running. In fact, the first prototype went through fourteen engines in just forty-four test flights and considering most of these flights were short hops of less than an hour, that is a very poor wear rate.

The test flights also threw up a number of issues with the aircraft, perhaps most spectacularly the very first time the XB-48 took to the air in June 1947. The flight reported a number of problems, which is to be expected, but then on landing the XB-48 had every single one of its tires blow out. Fortunately, the damage was comparatively minor, and no one was hurt but it was hardly a promising start.

Probably Martin’s biggest mistake after the novel jet pods was in retaining the straight wing.

Between this and the problematic powerplant the XB-48 failed to achieve the required speeds and performance, having a recorded top speed of 516mph (830km/h) at 20,000ft (6,096m).

This was in sharp contrast to its rival, the Boeing XB-47. Engineers at that company had completely redesigned the wings on this aircraft after they had the chance to examine captured German data soon after the war. Combined with a more conventional engine placement, the XB-47 was some 74mph (119km/h) faster than the XB-48, a considerable margin.

Martin suffered a further indignity when they were three months late in delivering the second XB-48 prototype to the USAF, who stated that the aircraft had too many flaws and that Martin could either rectify them or else pay a $25,000 penalty. As this was in October 1948 Martin chose to pay the fine, because by this point the choice had already been made and the Boeing had been selected, becoming the B-47 Stratojet.

And that was pretty much the end of the XB-48, an interesting example of an aircraft being both too innovative, yet also not enough. After all, if Martin had gone for the simplest option, like North American, and pushed out a more conventional design quickly, it is entirely possible that the XB-48 could have been adopted. But by not embracing modern developments more fully, as Boeing did, they ended up with an aircraft that was, quite frankly, thoroughly outperformed.

Of course, this is all obvious to see in hindsight, and had the B-47 suffered some terrible development issue it would have been the more staid and conventional XB-48 that would have provided the USAF with their primary nuclear bomber during the early days of the Cold War.

Interestingly Martin’s subsequent bomber design, the XB-51, was extremely innovative and, notably, taking shape at the same time as the XB-48 was failing to impress, so I think it is safe to conclude that the company took the lessons to heart.

But as for the XB-48s, well they were retained for a brief period for conducting test on de-icing systems, with the first aircraft being cannibalized to provide spares to the second, but by 1951 the need for the final example had passed and it was sent to the Aberdeen Proving Ground and underwent static ground tests until it was destroyed.


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