Douglas A/B-26 Invader

Prototypes - On Mark Engineering

HOME | SUB INDEX | Preface | Features | Site Navigation | Reg'n cross ref. | S/No's & Prod'n codes | MILITARY VARIANTS - History, Data & Photos | CIVIL VARIANTS - History, Data & Photos | About the author/Contact | Info Req'd | LATEST

On Mark Engineering
Written by Richard E Fulwiler

In the early 1950s, Grand Central Air Terminal, Glendale, California, was a hot-bed of ex-military aircraft conversion and modification.

This historic airport (which exists almost completely intact but is in threat of demolition by the Disney Corporation which now owns the property) was America's first west coast transcontinental terminal and over the years (it closed in 1959) had seen numerous historic aviation firsts.

Grand Central Aircraft Company executives came up with the idea to modify an Invader to carry passengers. Several Invaders were completed when the employees split off and established On Mark Engineering at nearby Van Nuys Airport. The company obtained a Supplemental Type Certificate (STC) and began to produce a number of conversions.

With conversion production getting underway in the late 1950s, the company created first the Executive, becoming the Marketeer in 1957, which were the unpressurized variants. Most had the rear spar carry-through (which limited cabin access) replaced by a strong and efficient circumferential "ring" spar of On Mark's patented design. By removing most of the original interior bulkheads and adding the ring spar, provided additional room for passengers in the new cabin space of the aircraft. The forward wing spar was not changed because of the magnitude of the re-engineering that would have been required, and would have altered the aircraft's design geometry and excellent flying qualities. With the wings mounted as with the original design, meant that crew members were left with the inconvenience of mounting the flight deck through a crawlway along the right side of the cabin beneath the forward spar. The customer would supply or either purchase a standard Invader which would be restructured in On Mark's spacious hangar. The cockpit would be removed and rebuilt to include dual controls, upgraded instruments and radios, a lengthened fiberglass nose for baggage and radar added, and, more importantly, the cabin was built to customer specifications, and added a number of large windows. The standard A-26 curved windshield design would be retained, with the upper canopy clamshells either painted or skinned over. Air stair doors were installed into the belly, or right side of the fuselage. Also offered, were distinctive 165 gallon wing tip fuel tanks added to increase range, and upgraded P&W R-2800's.

Next came, perhaps the best-known of the civilian Invader conversions, the On Mark Marksman series in 1961. The Marksman was the pressurized variant and required a special Supplemental Type Certificate (STC). The airframes were restructured with a greatly altered fuselage, DC-6 / 7 canopy structure with heated windshields, radio/navigation/radar improvements, ring spar, lavatory and food service consoles, soundproofing, air conditioning, increased area vertical tail, long nose, wingtip fuel tanks, improved anti-skid brakes, deicing, and numerous other upgrades and systems included as standard. The pressurized Marksman series were offered in three versions; Marksman A, B, and C. The Marksman A had 2100 hp R-2800-83AM3 engines; the Marksman B had 2100 hp R-2800-83AM4A engines; whereas the premier Marksman C had a raised cabin top that provided a 6 foot walk-through full length cabin, 2500 hp R-2800-CB-16 / -17s and added internal wing auxiliary fuel tanks. Only 15 percent of the original fuselage remained, and dependant on customer options, were designed to carry from 6 to 8 passengers.

The base price of the Marksman A was $257,430, up to $361,492 for the Marksman C

The On Mark Marksman was regarded as the best of the A-26 Invader conversions, providing up to eight business executives and a crew of two, an all-weather aircraft that was able to cruise up to 25,000 feet at 325 - 365 miles per hour with a range of  between 1200 and 2500 miles with normal reserves. At 20,000 feet, the pressurized, air-conditioned cabin was at a comfortable 7,500 feet.

On Mark also built a model and partial mock-up of the Model 450 which was a greatly modified pressurized aircraft fitted with Allison 501 D turboprops but, after a lot of deliberation, the project was shut down. Oddly, there is no exact record of the number of converted Invaders built by On Mark but its thought to be up to 50 aircraft. Also, the reign of the On Mark aircraft was short - a new generation of business jets led by the Lockheed JetStar and Lear Jet quickly dominated the market and the On Marks were purchased in the late 1960s and 1970s by drug runners who respected the type's long range and load carrying abilities. The following is a quick look at On Mark Invader conversions.

On Mark  -  Marketeer


On Mark Marketeer: Prototype serial number: 44-35326 ( c/n: 28605 )  Registered as N40Y ( 1957 )

Unpressurised version of the executive transport produced by On Mark Engineering on almost production-line basis.

After the airplane was stripped of its armament, it was entirely re-engineered, re-framed and altered. All surface metal of fuselage, wing and empennage structures were inspected. The aft wing-spar that passes through the fuselage was removed and was replaced by a ring of steel. Most of the partitions within the fuselage were removed to open up the cabin area. Skin, stringers and ribs were altered, added to and strengthened.

Large 'picture windows' were installed, as was the side entrance. Thick Fiberglass insulation was packed in cabin and cockpit walls to quiet the cabin..

When changes in the structure were completed, the interior was finished off to customer's specification that included a lavatory and a seating capacity for up to seven passengers in clubroom comfort.

Extra fuel tankage in the wings, tip tanks, an auxiliary power unit (APU), advanced radios, radar, and 2,500 hp (1865 kW)  R-2800-CB-16 / -17 radials were often installed to customer specifications.


On Mark Engineering ( Marketeer ) Modification Details and Prices – May, 1962

The following is transcribed from a May 1962 price list from On Mark Engineering for civilian conversion of B-26 Invaders.

NOTE: Many of the prices listed below are based on performing the modification at the time of complete aircraft conversion and consequently would be slightly higher if performed separately. Price adjustment would depend on the configuration of the customer’s aircraft.

1.  Fuselage and spar conversion. Install circumferential rear wing spar. Install Airstair door RH side. Install 8 windows ( 2 picture windows ). Recontour fuselage top &bottom. Modify wiring, plumbing, cables, floor, etc. $58,000

2.  Fuselage conversion. Install belly ladder door. Install 8 windows ( 2 picture windows ). Recontour fuselage top only. Modify wiring, plumbing, cables, floor, etc. $38,000

3.  Custom Interior. $17,000.

4.  Deluxe Interior. $25,000.

5.  Install Wing Tip Tanks ( 165 gal. each ). Modify wing plumbing and valves. Install boost pumps and dump valves. $16,710.

6.  Install Pliocell Wing Tanks ( 100 gallons each ). Modify wing, plumbing and valves. $10,500.

7.  Replace existing self-sealing main and aux. fuel tanks with lightweight Pliocell tanks. ( reduces airplane weight 450 lbs. ) $5,200. With top filler necks ( adds 90 gal. usable fuel ). Includes A.D. Note compliance. $6,000.

8.  Install 103” Plastic Nose. Includes ladder, baggage door. Provides for 1,000 lbs. baggage and radar. $10,500.

9.  Install dual controls. $5,500.

10.  Install Co-Pilot Brake Pedals. $990.

11.  Install 100,000 B.T.U. Heater and Ducting. $3,900.

12.  Install Custom Instrument Panel. Includes complete set flight instruments for co-pilot. Includes custom glare shield for radio controls. $4,250.

13.  Install Scott High Pressure Oxygen System. $1,800.

14.   Install DC-6 Wheels and Brakes. $4,850. In kit form. $4,450.

15.   Install Hytrol Anti-skid braking system. $5,250.

16.   Chrome Plate landing Gear struts. $1,200.

17.   Install Tip Tank Landing Lights. $1,050.

18.   Install De-icer Boots all Surfaces. ( Goodrich high-pressure “stick-on” type ) $8,850.

19.   Install Long all-metal rudder. Includes modified tail cone and vortex generators. $9,750.

20.   Install Nose Wheel Steering. Choice of rudder pedal or aux. wheel control. $1,995. In kit form $1,550.

21.   “Ceconite” covered Control Surfaces-exchange. $1,800. With new draft curtains $2,150. Installed complete $2,450.

22.   Modify Canopy. Metalized top and install double glass $1,750.

23.   Modify Cockpit Plumbing and Structure. Provide space for observer seat $3,100.

24.   Install Refrigerated Air Conditioning. Operates in flight or on the ground $5,000.

25.   Install Auxiliary Power Unit: New 105 ampere Homelite APU $2,450. Surplus 70 ampere APU $1,700.

26.   Custom Exterior Paint – paint only. $3,850. Strip and clean $960. Seal Exterior $1,100.

27.   Install 20-gallon Engine Alcohol Tank. $870.

28.   Install Fire Warning and Fire Extinguishing System with Firewall Shut-off Valves. $2,650.

29.   Install Modified Short Metal Nose. Includes ladder baggage door. $2,500.

30.   Install New Bendix Weather Radar. Includes Sperry Gyro Antenna Stabilization ( 103” Plastic nose required ) $18,500.

31.   Install Overhauled, Certified Sperry A-12 Autopilot unit $18,500.

32.   Install P&W R-2800 “C” Series engines with Hamilton Standard 33E60 High-activity propellers and autofeather. Labor and installation material only. Engines and propellers priced separately. $15,450.

33.   Install P&W R-2800 “CB” Series engines with Hamilton Standard 33E60 High-activity propellers and autofeather. Labor and installation material only. Engines and propellers priced separately. Includes ADI installation. $16,800.

34.   Install propeller spinners and afterbodies. Includes cowling mod. For inside carb. air scoops. On request.

35.   Install Hamilton Standard 43E60 reversing propellers on  “C” or “CB” engines. Labor and installation material only. Propellers priced separately. $4,800.

36.   Install Booster Tab Rudder and Vortex Generators ( with exchange rudder ). Lowers Vmc to 118 mph C.A.S. ( standard engines ). Includes new Ceconite cover and matching paint $3,950. In kit form $3,750.

37.   Flap modification for increased flap extension speed. 25 degrees flaps may be extended at 250 mph I.A.S. $265. In kit form $145.


See feature by Richard E. Fulwiler

On Mark  -  Marksman A


On Mark Marksman A: Prototype serial Number: 43-22416 ( c/n: 18563 )  Registered as N100Y ( early 1961 )

Pressurized, air conditioned version of the executive transport produced by On Mark. Cockpit windshields and side windows were flat panels from Douglas DC-6 / -7 airliners, while cabin windows became rounded-corner squares. Extra sound deadening materials were used to further quiet the cabin. 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 R-2800-83AM4A radials.

On Mark  -  Marksman C


On Mark Marksman C: Prototype Serial number: 44-34761  ( c/n: 28040 )  Registered as N400E ( late 1961 )
Vastly modified, premier version of the pressurized Marksman series. Cabin interior volume was increased by raising the fuselage top to provide a 6' 1" full length "walk-through" height. All "C" model Marksmans were powered by high output Pratt and Whitney R-2800-CB-16 / -17 radials developing 2,500 hp (1865 kW) for take-off with water injection. Also standard were fully reversible, broad chord Hamilton Standard propellers, and a "707" type anti-skid braking system. The Marksman C was the ultimate in executive transport only for a brief time until executive jets became available.

The early radar-equipped Marketeers had fiberglass nosecaps that were round-ended conical sections, but otherwise "B" model gun-nose profile. But when they went to the 103" nose, it was a extended, sharply tapered, all fiberglass unit that mounted on fuselage station "0". All Marksmans and many of the Marketeers were equipped with the 103" nose.

On Mark – Marksman - Special Purpose conversion ( CIA )
Note: There is enough difference between the A,B and C prototypes to justify the inclusion of Marksman #7 ( N46598 ) into this section as a separate prototype.


On Mark Marksman Special Purpose: Prototype Serial number: 44-34415  ( c/n: 27693 )  Registered as N900V ( 1964 ), operationally as N46598
Modified further from the premier version of the pressurized Marksman C, the “Special Purpose” was contracted by the U.S. Government to be able to perform covert operations abroad as a civilian registered aircraft. Cabin interior volume was the same as the previous C model with the  6' 1" full length "walk-through" height, and the standard starboard side Airstair entry door.
The fuselage was altered further with the addition of a belly cargo door to eject material and/or personnel while in flight. The exact details of this door vary, some sources say it was a 3 foot square in the belly, while others say it was a ramp type similar to those found in the C-123 and C-130 cargo aircraft.
Executive interior furnishings were reportedly installed. Ordered were the standard Marksman C Pratt and Whitney R-2800-CB-17 radials developing 2,500 hp (1865 kW) for take-off with water Methanol injection (ADI) with matched fully reversible, broad chord Hamilton Standard 43E60 propellers, and  "707" type anti-skid braking system for short field operations.
This Marksman was special in another way. The standard 103” fiberglass nose was equipped with terrain-following radar. The only other aircraft in the period so equipped was the General Dynamics F-111 which became operational in 1967. It is possible that this Marksman was created as a flying test bed for the new technology in a operational situation, but at lower overall speeds. The aircraft was also equipped with the Sperry SP-40 Flight Control System ( Autopilot ) and a radar altimeter. With the autopilot coupled to the terrain following radar, this aircraft could be flown “ hands-off “ in total darkness at low altitudes getting steering guidance and obstacle avoidance from the flight control system.
This Marksman was known to have been operated in S.E. Asia in the late ‘60s by CIA front companies in conjunction with Air America and based at Udorn R.T.A.F.B.. Thailand. She was nicknamed the “BLUE GOOSE “ and was reportedly damaged beyond repair and scrapped while in Asia. A second Special Purpose Marksman was built to these specifications, however, not used operationally and never leaving the U.S.. That aircraft was also damaged and reportedly scrapped at Norton A.F.B., California sometime in the ‘70s. Very little of the “ truth “ has been officially released on these two aircraft and her utilization in the “ Secret War “.    (  This information was compiled from various previously published sources and is relying on the accuracy of those sources.  )



Relating to the above two photos: 

Air America base at Udorn R.T.A.F.B., Thailand.

The "Blue Goose" in  the top photo was parked on the pie-slice shaped ramp, ( in red ) just above the 4 roof peaked row of buildings ( hangars ) in the center of the above photo.

Direction to source for the above photo and narrative, was supplied
by Richard E. Fulwiler

After completing operations in South east Asia, Blue Goose returned to the US and was handed over to the Operational Evaluation & Training (OE & T) Squadron, like its sister ship N67623 that was to follow later. So the story goes, the On Mark B-26s: were picked up two from Intermountain. They gave them to the OE&T as no one wanted them. One was landed nose gear up at Norton and it is not known what became of it.

The B-26 was equiped with a DC-7 nose so two pilots could fly side by side. It had a place for an Electronic Warfare Officer over the wing box and in the back were seats and a back door for jumping and jump lights.

It seems that the Blue Goose did not survive with the OE&T for very long – probably because it had been damaged beyond repair in that accident. A former member of the maintenance personnel assigned to the 1198th OE&T Squadron recalls that he was involved in "the total dismantling of both of those aircraft. One in Thailand and the other at Norton and they were sold for scrap. They both had the same problem. Nose gear failure. This seems to have happened some time after the April 68 accident, as on 18 September 68, the registration of the aircraft was officially cancelled as "scrapped"


On Mark Marksman  “ Blue Goose “  Specifications


Manufacturer:   Douglas

Conversion:   On Mark  “Marksman”

Description: Pressurized Twin Engine Light Freighter or Executive Transport with rear, bottom cargo door

Engines:   2500 HP with Water Methanol Injection R2800 CB16/17

Usable fuel:   7500 lbs.   (1250 gal.)

Range (Dry Tanks):   1600 N.M.   (At  T.O. Gross Wt. 40,000 lbs.)

Range with 3 hour reserve (Loiter Power):   1150 N.M.  (Note:  Average range equals .215 N.M. per lb. of fuel used)

Normal cruise speed: 230 kts.

Gross WT.  (T.O.):   FAA rated 35,000 lbs.;  can be operated 40,000 lbs.

Empty WT: 27,148 lbs.

Cargo capacity:  4,000  lbs. / 120 cu. ft.

Passengers: Seven plus crew of three

Special purpose: Aerial resupply, low-level penetration



                1.    EDO  Loran

                2.    Doppler / Nav computer  (Dual)

                3.    TAS  Indicator

                4.    Dual  VOR / ILS

                5.    Dual  ADF

                6.    Dual  DME


                1.    HF   (CW hand key or voice)

                2.    SSB  with upper or lower side band,  AM,  CW modes

                3.    Dual  VHF

                4.    Transponder


                1.    Sperry  SP-40  Auto pilot  Flight Control System

                2.    Periscopic sextant and mount

                3.    Radar Altimeter

                4    Terrain following radar with:

                       a Mapping mode (PPI) sector only

                       b Automatic Terrain Following and Autopilot

                       c Manual Terrain Avoidance  (Vertical and Lateral)

                5.    “BSTR” electronic jamming  (ground)

                6.    “ATIR”  electronic jamming  (air)

                7.    “ Vector Sector”  electronic position (air)


Recreation  of original document   


Description of the On Mark attached to George Doole’s letter of 28 March 67 (in: UTD/Bisson/B5 microfilm reel 4)


Recreated from the above source material in readable text format by Richard E. Fulwiler, 18 December 09



This B26, N46598, was painted blue with white trim stripes and nicknamed the "Blue Goose". It was also affectionately referred to as the "Blivit" which implied something-stuffed full of more then it could hold. This was true, as it had everything from A to Z when it came to electronic equipment and then some!

The Blue Goose was considerably different from the WWII Douglas A26 Invader; a medium bomber whose designation was changed to B26 in the 1950's. This hybrid B26, arrived with many of the modifications taken from the OnMark Company's corporate Marksman C and a few from the OnMark USAF B26K attack bomber also known as Nimrods when flown at Nakhon Phanom in eastern Thailand.

The similar modifications consisted of wing tip tanks, copilot instruments and controls, and enlarged rudder for better control, oversized anti-skid brakes and an air stair door on the starboard side. The engines had reversible props and with water injection, gave each P&W engine 2500 hp on take off. Also our Blue Goose had the bomb bay doors removed and the addition of a large couch with several seats as in the corporate Marketeer modification.

The paramount changes featured in this B-26 were Terrain Following Radar; precise navigational gear and a cargo drop ramp. These additions enabled "598" to deliver supplies at night, at low level and in most any kind of weather. The TFR was new at that time. In fact, the only aircraft to have it was the new USAF F111, an all weather twin jet fighter bomber not yet deployed in SEA. Installed in the B-26 nose, this unique radar enabled it to fly low at night over any terrain getting to the drop area and away from it as quickly as possible. The auto pilot could be coupled to the TFR and the navigational equipment was checked before and after each flight with exceptionally accurate results.

The other major feature in AA's B-26 was its cargo drop ramp, similar to that in the C123 and the Caribou aircraft, but much smaller. A pallet of approximately 500 pounds of supplies was pushed out of the B-26's cargo ramp by the Air Freight Specialist (AFS) or as commonly called, the kicker.

Serial #: 44-34415
Construction #: 27693
Civil Registration:



  On Mark Marksman
Name: None
Status: Unknown



Last info:


Reg'd 12th July 1962 N500X gulf Air Inc, Miami

30th Jan 63 Intermountain Aviation

1964 On Mark Eng Corp for conversion to on Mark Marksman and re-registered N900V

15th feb 64 sold to Intermountain Aviation as N900V

Aug 64 Sold to Atlantic General Enterprises Inc

29th March 65 Sold back to Intermountain Aviation

3rd Feb 67 sold to Pan Aero Invest, Reno

5th Feb 67 Sold to Air America as N46598

Officially reregistered as N46598 on 11 April 67 it was ferried to South-East Asia in mid-April 67, then used for training at Udorn from the second half of April to June 67; seen at Udorn April 67 and July 67; between 31 May 67 and 11 June 67. the plane was then ferried from Savannakhet to Udorn and back to Savannakhet, and used for nightly low-level supply drops over Laos, but proved unsuitable for the task, because the aircraft was too fast and too similar to a B-26 bomber.The plane continued to fly for Air America, but just doing transportation flights.

N46598 was then sold to Overseas Aeromarine Inc, Seattle, on 30 March 68, and registered to them as N46598 on 4 April 68, flown to Takhli Royal Thai Air Force Base in mid-April 68; damaged on take-off from Takhli on the ferry flight to the USA in mid-April 68 and burned. The crash was caused by a anti-shimmy poppet valve in the nose strut. The plane had been parked in Laos for several months and had deteriorated badly.

Costs of these aircraft
  • On Mark Marksman A - $257,430.00
  • On Mark Marksman B - $314,715.00
  • On Mark Marksman C - $361,492.00
Note: These prices did not include for radio, electronics or sales tax


Early nose with radar (note the slight step in conical section)


103" Nose, often used by other Invader conversion companies such as Wold.
Clarification by Richard E. Fulwiler