In November 1944 the US Army Air Forces looked ahead to the future of jet technology in issuing a specification for a jet-powered bomber with a range of 3,000 miles, a service ceiling of 45,000 feet and a maximum speed of 550 mph. By January of the following year the requirements were increased with the necessary ability to carry specific types of large bombs in the USAAF inventory. Four companies would eventually come to submit designs that would reach the flying hardware stage- the North American XB-45 Tornado, the Convair XB-46, the Boeing XB-47 Stratojet, and the Martin XB-48. With the XB-45 and XB-46 being four-engined bombers, they were paired up to compete with each other for a production contract and the XB-47 and XB-48 both being six-engined bombers, ended up being paired up to compete as well. By December of 1945, the Glenn L. Martin Company in Baltimore signed a contract with the USAAF for its submission, the Model 223 which received the designation XB-48. A newer contract superseded the original contract which called for two XB-48 prototypes with a first flight date no later than the end of September 1947.
While the general layout of the Martin XB-48 was conventional (straight wings, for instance), there were many features on the XB-48 that were unique and ground-breaking for aircraft technology of the day. Since the wings were too slender to carry the main undercarriage, the XB-48 featured a bicycle undercarriage with outrigger wheels that retracted into the outer sections of the underwing jet nacelles. This landing gear arrangement was first tested on a modified Martin B-26 Marauder nicknamed the "Middle River Stump Jumper" and designated XB-26H. The other significant unique feature of the XB-48 was that in order to keep the wingspan reasonable, the three engine nacelles on each wing were grouped together to form a lifting surface in which the nacelles top surface was faired into the wing and contributed to the overall wing lift. The General Electric J35 engines had their own sub-nacelles with an air duct passing between each nacelle and exhausting out the back of the nacelle. The jetpipes were also adjustable via flaps that deflected the jet exhaust.
The first XB-48 made its first flight at the Middle River plant's airfield on 22 June 1947 and flew to NAS Patuxent River 80 miles away for more flight testing. The J35 engines proved to be one of the biggest headaches in the flight test program. The first XB-48 aircraft went through fourteen J35s in only 44 test flights! By this time the USAAF was an independent military branch as the United States Air Force and flight testing of both the XB-48 and the Boeing XB-47 showed that the Stratojet was clearly the superior aircraft thanks to its more powerful J47 engines and highly refined aerodynamics with its thin swept wing. Martin's XB-48 ended up being 50mph slower than its original design speed and as a result, the XB-48 program was canceled by the USAF in September 1948 with an order of the first production B-47A Stratojets.
However, the USAF did make enough funds available for the completion of the second XB-48 prototype and for its flight testing. The second XB-48 first flew on 16 October 1948, three months behind schedule. But the delay was of little significance since the USAF had already terminated the program. Martin then offered to modify the XB-48 design in 1949 with XT40 turboprops which would have been more capable than the B-50 Superfortress, but by this point the USAF was interested only in pure jet bombers, not to mention that the XT40 was a Navy-funded engine and in those days, intraservice rivalries played a significant role in weapons development. In March of that year, Martin was formally notified of the USAF's lack of interest in the XT40-powered version of the XB-48.
With the formal end of the flight test program in the summer of 1949, Martin elected to keep flying the XB-48 as test beds. The first aircraft would be used as a spares source to keep the second aircraft flying and test schedules were drawn up to test items like autopilot systems, engine cooling technologies and hydraulic equipment. In the end, though, even those tests got canceled and the second XB-48 ended up only flight testing a thermal de-icing system. In September 1951 the sole remaining XB-48 was flown to the Aberdeen Proving Ground in Maryland and static tested to destruction.
Source: American X & Y Planes: Volume 2; Experimental Aircraft Since 1945 (Crowood Aviation Series) by Kev Darling. The Crowood Press, 2010, p14-17.