XB-42 was a re-modified aircraft, previously known as XA-42. The Douglas Airplane Company came up with the XB-42 design in early 1943 after the US Army Air Force wanted a replacement for the XA-42. Back in 1939, General H. H. Arnold recommended that it is safer to fight the enemy with light bombers and bomb loads rather than depending on the machine guns of attack type aircrafts; this recommendation made the Douglas to carry out a study and the result was – development of XB-42.

The Douglas created the XB-42 slightly smaller than A 20 and A 26 airplanes as  the Army Air Forces (AAF) wanted modern light bombers to avoid using costly strategic bombers for tactical applications. The AAF wanted a smaller, more efficient, more economical, speedier, and longer range tactical bombardment aircraft; XB-42 was just the one. Acquisition of the XA-42 however was related to that of the B-29. The design proposal impressed the AAF quite a lot; so the ‘Letter Contract W535-ac-40188’ was approved on 25 June, 1943.

Most striking features of XB-42 were the clean aeronautical lines and the novel engine propeller arrangements. Douglas incorporated two Allison liquid cooled, reciprocating engines in its fuselage for eliminating the drag of large nacelles. Pusher type propellers were located in the empennage of the aircraft to activate the thrust disturbances. Win shafts of the aircraft connected its propellers to the forward located engines.

In September 1943, the AAF asked Douglas if jet engines could be added to one of the experimental aircraft, it was working on. Soon after that, the Materiel Command recommended that jet engines should be installed in the aircrafts. Douglas was quick on pointing out that development of a practically new aircraft would take much time. Douglas suggested that if it modifies one of the XB-42s, it would be much quicker and easier. The AAF agreed.

Douglas managed to design and build the XB-42 in the record time of less than a year. It was first flown on 6 May 1944. The flight was conducted over Palm Springs Army Air Base, California. The AAF's urge in jet propulsion was growing and development of jet bombers was strongly favored. So, the XB-42 modification as suggested by the manufacturer would not get underway before 1945, one year after the aircraft's first flight.

The XB-42 was the first AAF bomber during World War II that had conventional tractor propulsion. 

In test flights, stability of the XB-42 proved satisfactory, but controls were rather insufficient. During the making of the aircraft, the XB-42 took extra weight than the design suggested, resulting in not meeting maximum speed at altitude or the range. Much to the frustration, excessive vibration was reported from the engines and propellers when doors remained open.

In a routine fashion, the contract of June 1943 was replaced on 11 February 1944 by an authoritative contract stating the same identification (W535-ac-40188). This contract included a new provision that covered building an all jet version of the XB-42, later known as the XB-43. On 23 April 1945, another contract change notification authorized the manufacturer to convert the first XB-42 to the XB-42A configuration.

Testing of the second XB-42, on 1 August 1944, was another disappointment as the US Army Air Force figured out that its combat capability was no better than that of the first model. The aircraft however improved in speed and range. In a coast to coast flight in November 1945, XB-42 covered 2,295 miles in 5 hours and 17 minutes.

The second XB-42 unfortunately was destroyed on 16 December 1945, in an accident near Bolling Field, D.C. Major causes were the failure of the landing gear and fuel starvation. This loss prevented completion of a number of special tests, planed earlier.

The first XB-42 was tested by the manufacturer for some 129 hours, accumulated in 154 flights. The second test lasted for more than 65 hours, accrued in 57 flights. The US Air Force put in 14 hours of flight tests on the first XB-42, and 51 hours on the second one. The manufacturer Douglas tested the modified XB-42 (XB-42A) by for approximately 17 hours in 22 flights. The Air Force test flew the XB-42A only once, for just one hour. Both the manufacturer and the user reported that the aircraft met the contractual acceptance requirements.

The US Air Force ordered Douglas to work on modifying the XB-42 conversion in April 1945. The process was soon postponed because Westinghouse 19XB-2A Navy type jets were not delivered on time. Bureau of Aeronautics was responsible for the act. Douglas could test the modified XB-42 on March 1946. Unfortunately enough, this time, the aircraft's left engine failed. Douglas then installed new landing gear, internal and external fuel tanks and the auxiliary turbojet engines for a better show. In early 1947, Douglas finally announced that it solved all the problems.  On 27 May 1947, Douglas tested the XB-42A from Santa Monica to Muroc Army Airfield, California. But again, both the Allison engines and added jets proved unsatisfactory. To make things even worse, vibration tests on the XB-42A were stopped on 15 August, when the XB-42A made a hard landing in the tail low position, damaging the lower vertical stabilizer and lower rudder.

The manufacturer wanted to resume testing as soon as possible, but the Air Materiel Command decided that the new jet nacelles required modifications. The specific XB-42A was flown back to Santa Monica in late 1947. Douglas was by than convinced that XB-42A was a failure and thought that further studies and engineering over the aircraft would not be economical. The US Air Force on the other hand wanted an ‘ideal’ XB-42A. After hearing the Douglas argument, the US Air Force in August 1948 agreed to cancel the remainder of the XB-42A modification program, and to accept the aircraft “as it is”. On 24 September 1946, the first XB-42, was conditionally accepted and became the XB-42A on 19 August 1948. The second XB-42 was accepted and delivered on December 1945.

Both the XB-42 and XB-43 were acquired under the same contract, W535-ac-40188, for $13,682,095, including the contractor's fixed fee of $227,775.

The US Air Force felt that the XB-42A, with its exclusive features was a true ‘museum piece’. So it decided to keep the XB-42A in the National Air Museum Storage Activity in Park Ridge, Illinois. The aircraft was later transferred to the Smithsonian Institution in Washington, D.C. Again in April 1959, the fuselage of the XB-42A was moved to the Smithsonian's Suitland Annex, in Silver Hill, Maryland.