The On Mark B-26K Counter Invader was a major redesign of the Invader, produced in the mid 1960s for use in Vietnam. Work on the B-26K began in 1962, but was given more urgency after a series of wing failures in 1963 and early 1964 forced the Air Force to ground its fleet of elderly B-26s.
On Mark Engineers, of Van Nuys California, was chosen because it had developed some expertise in converted surplus B-26s into high speed commercial transport aircraft.
On Mark proposed an almost total rebuilt of the aircraft. The fuselage and tail would be remanufactured, the rudder replaced with a larger version and removable duel controls fitted as standard. The wing would be reinforced with steel straps running from tip to root, partly to solve the wing failure problem but more importantly to allow for the installation of eight hard points each capable of carrying 1,000lb of stores, bringing the total payload of the B-26 up to 12,000lb. More powerful 2,500hp Pratt & Whitney R-2800-103W engines with water injection were to be used, maintaining the aircrafts top speed even with all of the extra weight. The aircraft was also given 165 gallon fuel tanks at the wing tips and a full package of up-to-date airborne electronics.
The hard points allowed the B-26K to carry a wide range of armaments. They could carry XM75 grenade launchers, P-2 grenade dispensers, LAU-3A, MA3, Aero 6A and LAU-10A rockets, MA3 rocket adaptors, SUU12 .50-caliber machine gun pods and bombs of up to 1,000lb. The inboard pylons could also carry a 230 gallon drop tank.
The prototype YB-26K made its maiden flight on 28 January 1963. After a series of tests it was accepted by the Air Force, and in November 1963 On Mark were given a contract to convert forty B-26s to the new standard (using -52W engines instead of the -103W of the prototype). The last conversion for the Air Force was completed in April 1965, and the company also received orders for the same conversion from other operators of the B-26.
The B-26K entered combat with the 609th Special Operations Squadron, from bases in Thailand. This led to one final designation change. The Thai government was unwilling to let the Americans operate bomber aircraft from its air bases, so the B-26K became the A-26A, reusing the designation once allocated to the Invader night fighter.
Engine: Two x Pratt & Whitney R-2800-52W Radial Engines
Power: 2,500hp each
Wing span: 71ft 6in
Length: 51ft 7 3/16in
Empty Weight: 25,130lb
Maximum Weight: 39,250lb
Max Speed: 327mph
Service Ceiling: 30,500ft
Range: 2,700 miles
Armament: Eight .50in machine guns in nose
Bomb-load: 4,000lb internally and 8,000lb on eight under-wing hard points