On Mark Engineering
On Mark Engineering was an American aircraft remanufacturing company known for converting surplus Douglas A-26 Invaders into executive transports and counterinsurgency aircraft.
Last updated August 26, 2026
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Overview
On Mark Engineering was an American aircraft remanufacturing company established in 1954 at Van Nuys Airport in California. Rather than developing a completely new airframe, the company specialized in extending and adapting existing aircraft designs for civilian executive transport and military applications. Its best-known work involved surplus Douglas A-26 Invaders, the twin-engine attack aircraft that had served during the Second World War and the Korean War. On Mark developed two principal civil conversions from the A-26 platform. The Marketeer used an unpressurized fuselage and was configured as an executive transport. The Marksman represented a more ambitious conversion, incorporating a pressurized fuselage for improved passenger comfort and higher-altitude operation. These aircraft demonstrated how surplus military airframes could be repositioned for the postwar business-aviation market through structural rebuilding and cabin modification. The company also adapted the A-26 for renewed military service. On Mark converted one aircraft into the YB-26K prototype, which was tested as the basis for a modernized counterinsurgency aircraft. Following successful testing, the prototype and 39 additional aircraft were converted into B-26K Counter-Invaders. The type was later redesignated A-26A and used by the United States Air Force for counterinsurgency missions. The program therefore connected On Mark's civil remanufacturing expertise with a significant military modernization effort; 40 aircraft were produced in the B-26K/A-26A conversion program. On Mark also undertook work for Aero Spacelines on the prototype Pregnant Guppy, a highly enlarged transport conversion based on a Boeing 377 Stratocruiser. The aircraft was developed to carry outsized aerospace cargo, illustrating the company's ability to participate in unconventional airframe modification projects beyond the A-26 family. The available reference material emphasizes On Mark Engineering's aircraft conversions rather than a broad commercial product portfolio, corporate ownership structure, financial performance, or later operating history. The company is consequently best understood as a historically significant specialist aircraft remanufacturer whose identity was closely associated with the Marketeer, Marksman, B-26K Counter-Invader, and related conversion programs.
History
On Mark Engineering was founded in 1954 at Van Nuys Airport in California as an aircraft remanufacturing business. Its work reflected a postwar aviation market in which large numbers of military aircraft became available for civilian or renewed military use. By rebuilding existing airframes rather than designing entirely new aircraft, the company could create specialized aircraft for customers seeking executive transportation, military capability, or unusual cargo capacity. The Douglas A-26 Invader became the central platform of On Mark's product and engineering activity. The company developed the Marketeer, an executive conversion with an unpressurized fuselage, and the Marksman, a more extensively modified version featuring fuselage pressurization. These conversions transformed a former military attack aircraft into a business-aviation product. The projects also established the company's reputation for structural and systems modification of complex twin-engine aircraft. On Mark's most important military project began with the conversion of a single B-26 into the YB-26K prototype. The aircraft was evaluated for counterinsurgency operations, and successful testing led to a larger conversion program. The prototype was followed by 39 additional aircraft, producing a total of 40 B-26K Counter-Invaders. The aircraft were subsequently redesignated A-26A and operated by the United States Air Force in counterinsurgency missions. This program represented a major extension of the company's remanufacturing role, turning aging surplus aircraft into aircraft suitable for a new operational environment. The company also participated in the development of the Pregnant Guppy prototype for Aero Spacelines. This aircraft was created through the conversion of a Boeing 377 Stratocruiser into an unusually large cargo transport intended for outsized aerospace loads. Although the available material identifies the aircraft as a prototype rather than a broad production line, the project shows that On Mark's capabilities were not limited to the A-26 family. The reference material does not provide detailed information about On Mark Engineering's founders, ownership, workforce, revenue, later corporate structure, or exact cessation date. Its historical importance rests primarily on the aircraft it rebuilt and the specialized engineering programs it completed. The company is associated especially with the Marketeer and Marksman executive conversions, the 40-aircraft B-26K/A-26A military program, and the prototype Pregnant Guppy conversion.
- 1954
Company established at Van Nuys Airport
[1]On Mark Engineering was established at Van Nuys Airport in California as an aircraft remanufacturing company.
Development of the Marketeer
[1]The company converted surplus A-26 Invaders into Marketeer executive transports with unpressurized fuselages.
Development of the Marksman
[1]On Mark developed the Marksman, an A-26-based executive transport featuring fuselage pressurization.
YB-26K prototype conversion
[1]A single B-26 was converted into the YB-26K prototype for evaluation as a counterinsurgency aircraft.
B-26K Counter-Invader production
[1]After successful testing, the prototype and 39 additional aircraft were converted into B-26K Counter-Invaders, later redesignated A-26A.
Pregnant Guppy prototype conversion
[1]On Mark undertook conversion work on a Boeing 377 Stratocruiser that became the prototype Pregnant Guppy for Aero Spacelines.
Products and positioning
A specialist aircraft remanufacturer focused on converting surplus or existing airframes into executive transports, military aircraft, and specialized cargo platforms.
Executive aircraft conversion
On Mark Executive
On Mark Executive was an executive-aircraft conversion associated with the company's work on surplus military airframes. The available reference identifies the product line but does not provide detailed specifications, production totals, cabin layouts, or a distinct launch date. It formed part of On Mark's effort to adapt former military aircraft for civilian business transportation.
Executive aircraft conversion
On Mark Marketeer
The Marketeer was an On Mark conversion of the Douglas A-26 Invader into an executive transport. Its fuselage remained unpressurized, distinguishing it from the more extensively modified Marksman. The aircraft demonstrated the company's approach of rebuilding surplus military airframes for civilian business aviation rather than producing a clean-sheet design.
Pressurized executive aircraft conversion
On Mark Marksman
The Marksman was a pressurized executive conversion based on the A-26 Invader. Its pressurized fuselage represented a greater level of modification than the unpressurized Marketeer and was intended to make the former military aircraft more suitable for executive transport operations. Detailed production and performance figures are not provided in the available reference.
Military aircraft conversion
B-26K Counter-Invader
The B-26K Counter-Invader was developed from the YB-26K prototype, itself created by converting a B-26. On Mark converted the prototype and 39 additional aircraft, for a total of 40. The aircraft were later redesignated A-26A and used by the United States Air Force for counterinsurgency missions.
Oversized cargo aircraft conversion
Aero Spacelines Pregnant Guppy prototype
On Mark performed conversion work on the prototype Pregnant Guppy for Aero Spacelines. The aircraft was based on a Boeing 377 Stratocruiser and modified into a large cargo transport for outsized loads. The reference identifies this aircraft as a prototype rather than documenting a wider On Mark production series.
Brand decisions
- Shift from surplus-aircraft rebuilding to counterinsurgency modernizationStrategy
Background
On Mark applied its A-26 conversion expertise to a military requirement for a renewed counterinsurgency aircraft.
Action taken
The company converted one B-26 into the YB-26K prototype and, after successful testing, completed 39 further conversions.
Aftermath
The resulting 40 aircraft became B-26K Counter-Invaders and were later redesignated A-26A for United States Air Force counterinsurgency operations.
- Development of the Pregnant Guppy prototypeProduct launch
Background
Aero Spacelines required an unusually large aircraft for transporting outsized aerospace cargo.
Action taken
On Mark undertook conversion work on a Boeing 377 Stratocruiser to create the Pregnant Guppy prototype.
Aftermath
The project expanded the range of specialized airframe-conversion work associated with On Mark Engineering beyond executive and military A-26 derivatives.
Sources2 sources
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