Douglas Invader

A US Army Air Corps requirement of 1940 for a multi-role light bomber called for fast low-level capability, with an alternative deployment from medium altitude for precision bombing attack, plus heavy defensive armament. To meet this specification Douglas proposed what was, in effect, a developed and enlarged version of the three-seat A-20 with Pratt & Whitney R-2800 Double Wasp engines. Three prototypes were ordered in June 1941, the first XA-26 making its initial flight on 10 July 1942. Like the A-20 it was of shoulder-wing configuration, the twin-engined powerplant was mounted in large underwing nacelles, and electrically- operated single-slotted trailing-edge flaps were provided. The hydraulically retractable landing gear was of the tricycle type: the main units were housed in the aft portion of the engine nacelles, but the nose unit rotated through 90' during retraction for the wheel to lie flush in the undersurface of the fuselage nose. Considerable thought had been devoted to satisfy the requirement for heavy armament, and the XA-26 had remotely-controlled dorsal and ventral turrets, each with two 12.7 mm (0.50 in) machine-guns, two similar guns in the fuselage nose, which had transparent panels and a bomb almer's position, and up to 3,000 lbs (1361 kg) of bombs could be accommodated in the bomb bay. To widen the assessment of suitable armament, the second (XA-26A) prototype was equipped as a night fighter, with AI (Airborne Interception) radar in the nose, four 12.7 mm (0.50 in) machine-guns in an upper turret and four 20 mm cannon carried in a ventral fairing. The third (XA-26B) prototype had only a single 75 mm cannon mounted in the fuselage nose.

A Douglas B-26K Invader - a post war counter insurgency and covert ops version

Extensive service trials showed that Douglas had provided an aircraft more than suitable for the intended role, and in so doing had exceeded every performance specification. Not only was the aircraft some 700 lbs (318 kg) below the design weight, but it could carry almost double the load of bombs which the US Army had considered necessary. The only area of delay in completing the trials and ordering the new attack aircraft into production came from the searching investigation to ensure the selection of the most effective armament. This was finalised for the first production attack version, designated A-26B, as six (later eight) 12.7 mm (0.50 in) machine-guns mounted in a solid nose; dorsal and ventral electrically operated and remotely controlled turrets each with two 12.7 mm (0.50 in) guns; and the bomb load increased to 4,000 lbs (1814 kg). The powerplant of these production aircraft comprised two R-2800-27 or -71 engines, and it is an interesting sidelight on the scale of the engine nacelles to appreciate that not only did the aft nacelle house the landing gear when retracted, but all of the engine accessory equipment; moreover, a door was provided in the fireproof bulkhead so that a mechanic could enter this area for maintenance of this equipment as required. Fast engine changes were made possible by the provision of quick release attachment for all pipes and wires connected to each engine.

Soon after entering service, the desirability of enhancing firepower for the ground-attack role was achieved by the provision of eight more 12.7 mm (0.50 in) machine-guns mounted in underwing packs, and modification of the upper turret so that it could he locked in position to fire forward, under control of the pilot, thus providing a total of 18 forward-firing guns. Not surprisingly, this combination of firepower and high speed (the Invader being one of the fastest bombers used by the USAAF during World War II) made the A-26 a highly effective aircraft.

Operational service of the Invader began on 19 November 1944, the type being deployed initially with 8th Air Force units operating in Europe: later it entered service in the Pacific, where its combination of firepower, speed and fairly useful range made it a valuable close-support aircraft during the last of the bitterly contested landings as the Allies hopped from one island to another en route to Japan's home islands.

In early 1945 the A-26Bs operating in the Pacific were joined by a new version, designated A-26C, in which some fairly extensive changes had been made. These included reinstatement of the bomb-aimer's position in the fuselage nose, with the necessary transparent panels, two of the nose guns being retained, plus the four turret guns. The fuselage was widened to accommodate pilot and co-pilot/bomb aimer side-by-side, and dual controls were provided as standard.

Other variants included a small number of FA-26Cs, equipped with cameras for use in the reconnaissance role; one XA-26D, developed from an A-26B, and armed with eight nose-mounted machine-guns, four guns in the two turrets, and six guns mounted beneath the wings to provide a maximum of 16 forward-firing and two aft-facing defensive machine-guns. With this armament 750 A-26Ds were ordered, but none were produced as such, for VJ-Day contract cancellation came before any were built. Subsequently, however, in-service A-26Bs were modified retrospectively to this weapon standard. Other nose armament experiments which had been carried out with the A-26B included a 75 mm cannon in lieu of the six 12.7 mm (0.50 in) machine-guns; a 75 mm cannon plus two 12.7 mm (0.50 in) machine-guns; and two 37 mm cannon. The A-26E, a development of the A-26C, was also cancelled at VJ-Day and the one other wartime variant, the XA- 26F, was used to flight test a General Electric J31 turbojet engine, this being mounted in the aft fuselage.

The performance and potential of the A-26 Invader was such that it continued in USAAF/USAF service for many years after the end of the conflict for which it had been designed, the designation B-26 being adopted in June 1948, when the USAF discontinued the classification of aircraft in an Attack category. B-26s became early reinforcements for NATO forces in Europe, were involved in the Korean War from 27 June 1950, and were operational in Vietnam in 1962. In this latter conflict, a number of B-26s were converted to a new COIN (counter insurgency) configuration by On Mark Engineering, under the designation B-26K (later A-26A). Following conversion of a USAAF A-26B Invader to target tug configuration in 1945 (designation XJD-1), the US Navy acquired 140 JD-1s converted from A-26Cs, for operation by US Navy Squadrons VU-3-4/ -7 and -10. Some were converted subsequently for the launch and control of target drones.

Korean War

For Korean War combat operations, the Invader operated at considerably higher weights and with greater loads than had been achieved in World War II. For example, the B-26B mounted eight nose guns and had three guns in each wing with a total of 4000 rounds. The four turret guns with 500 rounds per gun and an offensive load of 4,000 lbs (1814 kg) of bombs could be carried in the internal bay and fourteen 5-inch HVARs under the wings. Two 165 US gallon fuel tanks or two 110 gallon napalm tanks could replace some of the HVARs. The gross weight often reached 38,500 lbs (16235 kg). The B-26C had the same underwing loads as the B and carried the same two defensive turrets. The C could carry H2S radar on an installation in the fuselage between the nose wheel and the bomb bay. The use of radar made it possible for the B-26C to carry out effective bombing attacks at night.
The A-26 has the honour of flying the last combat sortie of the Korean War, when, with 24 minutes before the cease fire went into effect on July 27, 1953 a B-26 of the 3rd BW dropped the last bombs of the Korean war.

Following the end of the Korean War, the A-26s began to be withdrawn from active service with TAC and replaced by jet-powered equipment such as the Martin B-57 and the Douglas B-66. The B-26 remained in service with the Air Force Reserve and the Air National Guard after having been retired by TAC (Tactical Air Command).

Vietnam War

When American forces first began to get involved in combat in Vietnam (at first only as advisers) B-26Bs and B-26Cs went into action in the counterinsurgency role with the Farm Gate detachment. Unfortunately, by this time the B-26s were nearing the end of their service lives and suffered from frequent wing failures, forcing them out of service. Those few that remained active were provided with a strengthening wing strap along the bottom of the wing spars to prevent catastrophic wing failures and prolong service life. The success of these modifications led the USAF to order a remanufactured version of the Invader from On Mark Engineering Company in Van Nuys, California that would be specifically adapted to the counterinsurgency role. The designation B-26K was applied and the name Counter Invader was chosen.

The B-26K Counter Invaders were delivered to the USAF between June 1964 and April 1965. They served with the 603rd Special Operations Squadron based at Lockbourne AFB and Hurlburt AFB in the operational training role, and with the 606th Air Commando Squadron (later renamed the 609th Special Operations Squadron) from Nakhon Phanom Air Base in Thailand. During the mid-1960s, Thailand did not permit the basing of bombers on its territory, and so the aircraft were reassigned the old attack designation of A-26A, thus bringing the Invader full-circle. The A-26As flew night interdiction missions over the Ho Chi Minh trail until they were phased out of service in November of 1969, finally bringing the era of Invader combat service with the USAF to a close.

Combat History

The USAAF issued a requirement for an attack aircraft in 1940, before it had information on World War II combat operations in Europe. Consequently, three prototypes were ordered in differing configurations. The Douglas XA-26 attack bomber with a bomb-aimer's position; the XA-26A heavily armed night-fighter; and the XA-26B attack aircraft with a 75 mm cannon. After flight testing and careful examination of reports from Europe and the Pacific, the A-26B Invader was ordered into production, and initial deliveries of the 1,355 built were made in April 1944.

The A-26B had six 12.7 mm (0.50 in) machine guns in the nose, remotely controlled dorsal and ventral turrets each with two 12.7 mm (0.50 in) guns, and up to 10 more 12.7 mm (0.50 in) guns in underwing and under fuselage packs. Heavily armoured and able to carry up to 4,000 lbs (1814 kg) of bombs. The A-26B was potentially a formidable weapon. Moreover, its two 2,000 hp (1491 kW) Pratt & Whitney R-2800 engines conferred a maximum speed of 355 mph (571 km/h), making the A-26 the fastest US bomber of World War II. Invaders remained in USAF service until well into the 1970s.

Missions with the 9th Air Force in Europe began in November 1944, and at the same time the type became operational in the Pacific. The A-26C with a bomb-aimer's position and only two guns in the nose entered service in 1945, but saw only limited use before World War II ended. A-26C production totalled 1,091. With little employment ahead of them, so far as anyone could see, one A-26B and one A-26C were converted to XJD-1 configuration, this pair being followed by 150 A-26Cs converted as target tugs for the US Navy with the designation JD-1. Some were converted later to launch and control missile test vehicles and drones under the designation JD-1D. These designations became UB-26J and DB-26J in 1962.

USAF A-26B and A-26C aircraft became B-26B and B-26C in 1948, and retained this designation until 1962. Both versions saw extensive service in the Korean War, and were again used in a counter-insurgency role in Vietnam. A special 'COIN' version with very heavy armament and extra power was developed by On Mark Engineering in 1963, a prototype being designated YB-26K and named Counter Invader. Subsequently about 70 B-26s were converted to B-26Kstandard, 40 later being redesignated A-26A. Some were deployed in Vietnam, and others supplied to were friendly nations under the Military Assistance Program. B-26s were used also for training (TB-26B and TB-26C), transport (CB-26B freighter and VB-26B staff transport), RPV control (DB-26C), night reconnaissance (FA-26C, from 1948 redesignated RB-26C) and missile guidance research (EB-26C). After the war, many A-26s were converted to executive, survey, photographic and even fire-fighting aircraft. Brief details of the two semi-production marks are given in the variants list.


XA-26C: A projected version with four 20 mm cannon in the nose. With the abandonment of the project, the C-suffix was reallocated to the transparent-nose version of the Invader.

XA-26D: A single prototype powered by two 2,100 hp (1567 kW) Chevrolet built R-2800-83 radials. The precursor of the proposed A-26D production series, of which 750 were cancelled after VJ-Day.

XA-26E: A single prototype powered by two 2,100 hp (1567 kW) R-2800-83 radial engines. The precursor of a planned production batch of 1,250 A-26E transparent-nose aircraft but these were cancelled after VJ-Day.

XA-26F: A single prototype (later redesignated XB-26F) with two R-2800-83 radials and a 1,600 hp (726 kg) thrust General Electric J31 turbojet in the tail to boost performance. Maximum speed was 700 km/h (435 mph) at 4570 m (15,000 ft). Insufficient performance gain to warrant production.

RB-26L: The designation RB-26L was assigned to two RB-26Cs that were modified in 1962 for night photography missions. These modifications were performed by General Dynamics at Fort Worth, with assistance from E-Systems at Greenville. The planes were equipped with state of the art reconnaissance systems such as cameras and associated equipment. One of them was equipped with a Reconofax Vi infrared aerial mapping system which could be used for night-time surveillance.

The two RB-26Ls were assigned to the Farm Gate detachment that was operating in South Vietnam, arriving at Bien Hoa in March of 1963. They replaced the RB-26Cs that had previously been operating in the reconnaissance role, and were for a while the only aircraft in South Vietnam with any real night reconnaissance capability, aside from the single RC-97 that was operating with Project Brave Bull. Unfortunately, the infrared system of the RB-26L was difficult to maintain in the damp and dusty climate of South Vietnam, and did not work as well as had been hoped.

A-26Z: Douglas designation for a proposed post-war model to have been built as the A-26G and A-26H with unglazed and glazed noses respectively. Improvements included a raised pilot's canopy and wingtip drop tanks.

Invader Mk I: Royal Air Force designation of 140 A-26Cs received under Lend-Lease.

On Mark Marketeer: Unpressurised version of the Marksman C.

On Mark Marksman A: Pressurised executive transport produced by On Mark Engineering on almost production-line basis. Powered by 2,100 hp (1567 kW) R-2800-83AM3 engines.

On Mark Marksman B: Similar to the Marksman A apart from the provision of wingtip tanks and R-2800-83AM4A radials.

On Mark Marksman C: Similar to the Marksman A apart from extra fuel tankage in the wings and 2,500 lbs (1865 kW) R-2800-CB-16117 radials.

Smith Biscayne 26: A high-speed transport version developed by the L.B. Smith Company and able to seat up to 15 passengers.

Smith Super 26: Standard Invader airframe converted with wingtip tanks and an executive interior.

Smith Tempo I: Unpressurised executive conversion with R-2800 B series engines.

Smith Tempo II: A pressurised executive conversion with a new fuselage which was 9 ft 7 1/2 in (2.93 m) longer than the standard and able to seat up to 13 passengers.

Specifications (Douglas A-26C Invader)

Type: Three Seat Light Attack Bomber

Design: Ed Heinemann

Manufacturer: Douglas Aircraft Company (The postwar B-26K was built by On Mark Engineering)

Powerplant: Two 2,000 hp (1492 kW) Pratt & Whitney R-2800-27/-71/-79 Double Wasp 18-cylinder two row radials. (B-26K) Two 2,500 hp (1865 kW) R-2800-103W radial engines.

Performance: Maximum speed 373 mph (600 km/h) at 10,000 ft (3050 m); cruising speed 284 mph (457 km/h); service ceiling 22,100 ft (6735 m); initial climb rate 2,000 ft (610 m) per minute.

Range: Range 1,400 miles (2253 km).

Weight: Empty 22,850 lbs (10365 kg) with a maximum take-off weight of 35,000 lbs (15876 kg).

Dimensions: Span 70 ft 0 in (21.34 m); length 51 ft 3 in (15.62 m); height 18 ft 3 in (5.56 m); wing area 540 sq ft (50.17 sq m).

Armament: (A-26B) Ten 12.7 mm (0.50 in) Brownings, six fixed in nose and two each in dorsal and ventral turrets and an internal bomb load of 4,000 lbs (1814 kg), later supplemented by underwing load of up to 2,000 lbs (907 kg). (A-26C) Similar but only two 12.7 mm (0.50 in) in nose. (B-26K, A-26A) Various nose configurations with up to eight 12.7 mm (0.50 in) or four 20 mm cannon, plus six 7.62 mm (0.30 in) guns in wings and an ordnance load of 4,000 lbs (1814 kg) in the bomb bay and 8,000 lbs (3629 kg) on eight outer-wing pylons. However, the actual load carried on combat missions was usually somewhat less in order to gain manoeuvrability and to reduce stress loads. A typical underwing load was a pair of SUU-025 flare dispensers, two LAU-3A rocket pods and four CBU-14 cluster bomb units. Later, the rockets and flares were often replaced by 500 lbs BLU-23 or 750-lbs BLU-37 finned napalm bombs. The M31 and M32 incendiary clusters could also be carried, as well as M34 and M35 incendiary bombs, M1A4 fragmentation clusters, M47 white phosphorus bombs, and CBU-24, -25, -29 and -49 cluster bomb units. General-purpose bombs such as the 250-lbs Mk. 81, the 500 lbs Mk. 82 and 750 lbs M117 could also be carried.

Variants: XA-26 (attack bomber), XA-26A (night fighter), XA-26B (attack bomber with 75 mm cannon), A-26B Invader (later B-26B), XA-26C, A-26C (later B-26C), XJD-1, JD-1, JD-1D (later UB-26J/DB-26J), XA-26D, XA-26E, XA-26F, YB-26K Counter Invader, B-26K, (40 later redesignated A-26A), A-26Z, TB-26B/TB-26C (trainers), CB-26B (cargo transport), VB-26B (staff transport), DB-26C (RPV control), FA-26C (later RB-26C), EB-26C (missile guidance control), Invader Mk I, On Mark Marketeer, On Mark Marksman A, On Mark Marksman B, On Mark Marksman C, Smith Biscayne 26, Smith Super 26, Smith Tempo I, Smith Tempo II.

Avionics: (XA-26A) Centimetric MIT AI-4 radar. (B-26K) AN/PVS2 Starlight scopes for enhanced night visibility on some aircraft only.

History: First flight (XA-26) 10 July 1942; service delivery December 1943; final delivery 2 January 1946; first flight of B-26K, February 1963; phased out of USAF service November 1969; last military aircraft (National Guard) was retired in 1972.

Operators: United States (USAAF & USN).