Army Air Corps's Experimental Engineering Section asked the Douglas Airplane Company to build such a plane that will be structurally stronger and quicker than the A-20. The requirements also included that the new plane should have shorter landing distances and takeoff as part of its defensive armament. The Air Corps wanted to replace all three existing planes - the A-20, the Martin B-26 Marauder, and the North American B-25 Mitchell with the new B-26 Invader. The 3 XA-26s (extensions on A-26) were ordered in 1941. 

The US Air Force’s experience from the battles in Europe and the Pacific area prompted it to give priority to ground attack over the multi-purpose light bomber requirements of 1940 and to design the third experimental plane, XB-26B ground attack, which has a 75-mm cannon nose mainly to destroy tanks. The plane's versatility was also not ignored; the authority planned to purchase 200 additional noses, each with six .50 caliber guns. These noses were fairly easy to install; field personnel could induct these within 24 hours. 

Mass production of XA-26 would not start before July 1943. Reasons behind the delay included lack of tooling and shortages of engineers; thus the Wright Field Production Division asked the Douglas to transfer about two thirds of the workforce listed on the C 742 project into the A-26. The Wright Field also asked the Douglas not to improve the plane’s crew comfort or any other activities. No studies on armament were carried out until the A-26 production's phase was advanced. 

In January 1943, the Douglas Airplane Company informed the US Air Force that it will not be able to meet the production schedule. Douglas however said that, it can start the production in October 1943; so it did.

Finally, in March 1944, Douglas handed over 21 A-26s planes to the US Air Force. General Arnold right away expressed his dissatisfaction and said, "One thing is sure, I want the A-26s for use in this war and not the next war." Maj. Gen. Oliver P Echols, Assistant Chief of Air Staff for Materiel, Maintenance, and Distribution, also expressed anger and blamed the Douglas for delaying. He said, "Little desire to manufacture the plane" by Douglas was the prime reason. He also explained that his department all along had urged the Douglas to place orders for tools and to find qualified subcontractors. However, the Western Procurement District, said that there were obvious reasons for the delay. First of all, the A-26 plane was completely different from any other existing ones; secondly, its delivery schedules were made before solving the designing and tooling problems; and last of all, 35 changes were made before completion. The authorities did not allow the production problems to affect the scheduled procurement of additional A-26s. The Under Secretary of War permitted only two supplemental orders along with the existing ones. 

The Douglas than took greater care to simplify the proceedings and maintenance on the A-26. The A-26 had a 70 foot wing span instead of 61 foot span of A-20 which was 30% lighter than the A-26. Moreover, 3 crewmen could exchange positions inside an A-26, which the A-20 did not have. The aluminum alloy monocoque engine of the plane was a combination of cowling and structure. It reduced the plane’s weight and eased up the engine installation process. The manufacturer also created slotted wing flap that had lower pitching movements. Additionally, a new type of entrance velocity cowling was installed in the planes which reduced the engine temperatures and stimulated less aerodynamic resistance. 

In mid 1944, the A-26 started its combat testing, after four of its flight assigned to the Fifth Air Force initiated operating in the Southwest Pacific. But Lt. Gen. George C. Kenney, Commanding General of the Far East Air Forces, grounded all these four planes after 175 hours of flying. He said, "We do not want the A-26 under any circumstances as a replacement for anything." Ironically, some four years back, when George C. Kenney was a colonel in charge of the Wright Field Production Division, he strongly backed the aircraft's development. His decision of grounding the A-26 is somewhat justified as its production had slipped several times and in the meantime, the B-25s and A-20s had proven to be satisfactory. In addition to these, canopy of A-26s was very poorly designed. To improve visibility, a new canopy was required. Without a good canopy, pilots had difficulties in forming in to required formations during low level tactics. At the end, Wright Field Production Division agreed that A-26 was not good enough to replace the than current types of bombers. Nonetheless, some A-26 operating in European region performed well.

The aircraft's marginal visibility undeniably needed more work but General Vandenberg thought that this plane was a satisfactory replacement for the B-26s and A-20s in Europe. Maj. Gen. Hoyt S. Vandenberg, Commanding General of the Ninth Air Force, on the other hand had some praise for the aircraft. Though there were mixed response on A-26 (both A-26As and A-26Bs), the authority decided to re-equip all B-25, B-26, and A-20 units with A-26s in November 1944. In the following month, two more contracts were approved. All these orders were supplemented in April 1945, bringing the total number of new A-26s to 4,000. The German surrender on 8 May 1945, however incited the production to 150 a month from 400 previously. The procurement orders of 1944 and 1945 were canceled.

The manufacturer Douglas then decided to adopt several long standing suggestions from General Arnold. Douglas established closer liaison between engineering personnel at Long Beach and Tulsa plant; employed qualified personnel for its two plants; and increased the stations at its production lines. These changed led to modifications of the aircraft for improving its effectiveness. The designers devised an ‘all purpose gun nose’ and made changes in the landing gear. The new A-26s (named A-26Cs) came into production lines in January 1945. It featured an enlarged, raised canopy that increased visibility for the pilots. 

The Ninth Bombardment Division said that once its pilots got familiar with the A-26, they liked it better than any other aircrafts. Even General Kenney agreed that improved A-26s with the 8 gun nose were acceptable replacements for the A-20s and B-25s. Deficiencies of canopy frosting, faulty brakes were there but the engineers were trying to overcome all of those in quick time and a substantial progress was achieved. Production of the A-26 was completed within 1945 and the last aircraft was delivered in early 1946.

In June 1948, the Martin B-26 Marauder was grounded and Douglas A-26 became the B-26. This designation represented its actual role as it was truly a bomber used by the United States Air Force and the Tactical Air Command. Overall, The Army Air Forces received 2,451 A-26s and canceled orders of over 4,000. The first 9 planes of this type were produced in El Segundo, California. The rest consisting of A-26Bs and A-26Cs, were manufactured in Long Beach and Tulsa. The Tulsa plant in total produced 1,086 of the 1,091 A-26Cs.

When the Korean conflict started on 25 June 1950, the B-26 went back into action with its role of a bomber. In the early days, its targets were North Korean troop concentrations, tanks, guns, supply elements, railway yards and bridges. Operational area was south of the 38th parallel. The first combat strike into North Korea was flown in 1950. Immediate results of B-26 in the Korean War nevertheless were rather disappointing mainly because of bad weather. Low sunlight also reduced B-26’s effectiveness. Engine failures and various mechanical deficiencies were also notified. Moreover, as the combat continued, other problems came in view. The B-26 did not have any electronic countermeasures capability and could not carry various types of new armament and guidance systems. On the evening of 26 July 1953, just a day before the Korean armistice agreement was signed, a B-26 dropped the last bombs of the Korean conflict.

From the early days of air-combats, the Far East Air Forces achieved air dominance against an enemy that offered little daylight air opposition on strategic or tactical operations. But situation changed at nights. Commanders were forced to use a portion of their day force for night operations, and the 3d Bombardment Wing's B-26s, which could be used for night duty, gained more importance in the force. The B-26s suffered massive losses during the battle as their errands increased at the maximum levels. Despite their confines, the B-26s achieved a distinguished record in air-combat history. 

Special modifications were in force after B-26's ineffectiveness was shown in Korean wars. The Air Staff in 1952, decided to include more sophisticated electronic equipments into the B-26s. As consequences, some B-26s were given additional components to perform electronic reconnaissance and weather reconnaissance missions. But even then, usefulness of the outmoded B-26 was declining. Additionally, too many configurations (16 in the US and about 14 in the Far East and Europe) created supply and maintenance problems. In mid 1953, authority approved the last modification to attempt standardizing most B-26s into a few basic configurations.

After the emergence of the Martin B-57, B-26s started leaving the Air Force's active missions by the late 1954. The flights of the B-26s were withdrawn from service in Air Force Reserve and Air National Guard units in 1958.

During the Vietnam War, President John F. Kennedy's policy enforced delivery of additional equipment to the South Vietnamese. Fixed wing aircraft were in short supply, so B-26s were modified for special combat missions there. Modified B-26s reached South Vietnam in the fall of 1961. Once in action, they accomplished a variety of tasks including standard bombing operations and close air support attacks to visual and photo reconnaissance missions. During mid 1962, the B-26's role in the conflict was expanded even further. Several B-26s received additional modifications for performing night photo operations and intelligence gathering duties.

Despite changes and the improvements, the B-26 actually belonged to a type that had been declared obsolete during the Korean War in the 1950s’. The maintenance became difficult due to the operating conditions of Southeast Asia and due to combination of old age & hard usage. The aircrafts became more vulnerable to enemy ground fire at times, and most flights were subject to restrictions to avoid excessive wing stress. Just the same, losses arise at flights that were directly due to structural fatigue. 

All B-26s were grounded and withdrawn from Southeast Asia after an aircraft crashed due to winks broke off in August 1963 and another failed during an air combat in February 1964. Yet, this act could not end the aircraft's air battle. Forty B-26s returned to the battle in mid 1966 after being named as B-26Ks. 

Van Nuys, California based On Mark Engineering Company’s modifications were quite extensive. The contract was worth $16 million. The contract involved a complete transformation rather than modifications. The B-26K differed from the basic design. Both of its turrets were removed; the B-26's R-2800 79s were replaced by R-2800 52W engines; the wings had been reinforced by the addition of steel straps both on the top and bottom of the spars; the propellers, wheels, brakes, and rudder were changed; permanent wing tip tanks were added; instrument panel and electronics were new; 8 wing pylons were included with other countless minor changes. The B-26K turned out to be a tactical bomber for special environments, mounted with rocket pods and guns pods and capable of being fitted with photographic reconnaissance components along with and other sensors. 

The invigorated B-26K quickly proved itself to be an effective hunter and destroyer of vehicles. Its linger capability enabled it to locate and attack an enemy hidden in jungle or by bad sunlight. Most A-26As therefore stayed in Southeast Asia for about three years. Its last combat mission was flown in November 1969. 

By 1972, regardless of its label, none of the old B-26s continued service in the US Air Force's active inventory line or the Air National Guard.

B-26 Invader Specifications

References

• B-26 Invader Encyclopedia of US Air Force Aircraft and Missile Systems Volume II by Marcelle Size Knaack