The War Department’s urge of incorporating heavier aircraft with gross weights ranging from 80,000 to more than 200,000 pounds made manufacturer Martin to propose the design the XB-48.

The Army Air Forces (AAF) was wise enough to realize that this particular project could be fraught with difficulties; so it asked the Materiel Command and Air Services Command to go on with a ‘phased basis’ contract; so that it can be readily terminated upon completion of any one stage of development. The XB-48 was the first US jet bomber. 

On 15 August, the procedure of designing and building was formalized. At the same time, Materiel Command and Air Services Command merged to form the AAF ‘Technical Service Command’. This same institute on 9 March 1946 became the Air Materiel Command. 

The AAF on November 1944, issued military characteristics for a bomber with a minimum acceptable range of 2,500 miles and an average range of 3,000 miles; a service ceiling of 45,000 feet - minimums acceptable 40,000; a tactical operating altitude of 40,000 feet - 35,000 feet; and an high speed of 550 and average speed of 450 miles per hour. The characteristics were modified later on 29 January 1945, only to ensure that such aircrafts will be used in carrying specific types of bombs, including the conventional M-121, a 10,000-pound bomb.

Martin proposed in 9 December 1944, that it will go along with the Air Technical Service Command’ requirements and will build such bomber. The AAF approved it on 29 December. The agreement covered certain engineering services and completion. Tentative costs were set at $574,826. This contract was replaced on 27 March 1945 by a definitive contract, which reduced estimated costs to $569,252, including Martin's fixed-fee of $16,500. On June 1945, Martin submitted a proposal for 1 stripped and 1, 2, or 3 complete XB-48s. The Air Technical Service Command noticed that the estimated cost of $80.09 per pound for the XB-48 compared favorably to the $105.68 for the XB-45. But still, the AAF remained dissatisfied because the XB-48's engineering seriously lagged behind the contemporary XB-45 and XB-46. Despite this fallback - the XB-48 program survived. 

On March 1946, the manufacturer introduced a new proposal and offered to furnish 1 stripped and 1 complete XB-48 for about $10 million. This proposal was bashed on a ‘fixed-price’ rather instead of a ‘cost-plus-fixed-fee’ basis. After many negotiations, the AAF approved the contract -W33-038 ac-13492- on 13 December 1946, with an estimated future cost of $10.9 million. The new contract promised 2 XB-48s, spare parts, and a bomb-bay mockup; the first XB-48 was to be flight tested and delivered by 30 September 1947; the second one by 30 June 1948 and all wind tunnel tests were to be completed by 1 January 1947.

Development did not go as promised. It was delayed by engine difficulties. General Electric was in charge of the turbojet engines and took time to get them to operate properly. The first XB48 was powered by 6 J35-GE-7 (TG-180-B1) engines; the second by 6 J35-GE-9s (TG-180-Cls).

All-metal, high-wing XB-48 had many special features. Firstly, it had a tandem landing gear necessitated by the airplane's wings. This bicycle-type landing gear was later used by other jet bombers as well, including the B-47. Other features included the number of engines, 6 as compared to 4 on the other proposed medium bombers and the adjustable tail pipes on the engines. The 3-crew arrangement was also unusual. The pilot and co-pilot of the aircraft seated in tandem under a canopy-type in-closure, just like in high-speed fighter planes, while the bombardier-navigator seated in the aircraft's nose. The aircraft was incorporated with retractable bomb-bay doors to carry the "Grand Slam" bombs, as well as the cumbersome atomic bombs of that time.

On 22 June 1947, the XB-48 made its first flight. The flight took off from Martin's airfield at Baltimore, Maryland and landed 80 miles away at the Patuxent Naval Air Station. The 38-minute flight indicated several faults. The Martin pilot discovered that at 10,000 feet, the right spoiler aileron of the aircraft snapped up too rapidly. Making things worse, on landing, the XB-48 drifted across the runway. Rudder steering was attempted, but was ineffective with the full use of brakes. Soon after, the brakes overheated and stopped working. Though the aircraft came to a halt with no damage, its tires were worn through.  The second test fly was scheduled on July 1948, but did not fly until October 1948. The 30-minute flight went well. 

Martin pilots in total tested the first XB-48, 52 times, for 41 hours; the US Air Force, 50 times for 64 hours. The second XB-48 was tested for 14 hours, accumulated in 15 flights; the Air Force, 49 hours, reached in 25 flights. Results of the test program revealed that the aircraft did not meet the Martin guarantees. Firstly, it was 14,000 pounds overweight; the nose wheel was too sensitive to work with; turbulence occurred in the bomb bay when the doors were open; and the metal chip shattered the hydraulic system. The US Air Force asked Martin to eliminate all flaws or to pay a lump-sum penalty of $25,000. Martin agreed to pay the penalty.

On 26 October 1948, the US Air Force conditionally accepted the first XB-4. The second XB-48, also conditionally accepted on 26 October 1948 after Martin completed a range of modifications in the aircraft. 

In the spring of 1948, the US Air Force conducted an evaluation to determine which of the 2 planes between XB-48 and XB-47 could best satisfy its urgent need for a high-speed, high-altitude medium bomber. The evaluation confirmed that XB-47’s performance was appreciably better than that of the XB-48. It was also apparent that the XB-47 design provided possibilities for growth which surpassed those of the XB-48. The XB-48 apparently lost the battle and saw the program officially ended in September 1948. Martin however, did not gave up and in 1949, proposed to modify the second XB-48 by removing the J35 engines and nacelles and installing 4 XT-40A propeller turbines in new and repositioned nacelles, at an estimated cost of $1.5 million. The reconfigured XB48 would become a prototype of the Martin Model 247-1, an airplane that could compete with the B-47, B-50, and B-54. Though, Model 247-1's performance looked good on paper, the Air Force did not believe the proposed reconfiguration could be accomplished for the amount of money estimated. Additionally, its XT-40A turboprop was a Navy-developed engine, it was doubtful that Martin could obtain enough engines to complete the reconfiguration on schedule. Also, senior Air Force officials believed that turbojet aircraft offered greater promise than turboprop installations. On 31 March 1949, Martin was formally informed that that the Model 247-1 was a dead issue.

In the fall of 1949, the first XB-48 was dismantled to provide parts for the second XB-48. The second one was used in many tests, including tests on the F-1 autopilot, jet engine cooling system, and a hydraulic system for jet engines. It was then used as a test-bed for "bad-weather" flight items; but completion of the thermal anti-icing survey test program in mid-1951 cemented the end of it. 

In September 1949, the XB-48 was flown to Phillips Field, Aberdeen Proving Ground, Maryland, for testing. Few days latter, it was totally destroyed.