Even though the Lockheed 12 had won the government's feeder airliner competition, most of the airlines rejected it, and very few Lockheed 12s were used as airliners.
One notable airline user was the newly renamed Continental Air Lines, which had a fleet of three Lockheed 12s that ran on its route between Denver, Colorado and El Paso, Texas in the late 1930s.
Another was British West Indian Airways Ltd., which flew the Lockheed 12 on Caribbean routes in the Lesser Antilles during the mid-1940s.

The Lockheed 12 proved much more popular as a transport for company executives or government officials.
Oil and steel companies were among the major users.
A number were purchased as military staff transports by the United States Army Air Corps, which designated the type as the C-40, and by the United States Navy, which used the designation JO, or in one peculiar case, R3O-2.
With the arrival of World War II, many civilian Lockheed 12s were requisitioned by the U.S. Army and Navy, Britain's Royal Air Force, and the Royal Canadian Air Force.

Two civil Lockheed 12s ordered by British Airways Ltd. were actually intended for covert military reconnaissance flights.
Sidney Cotton modified these aircraft for aerial photography and in civilian guise, overflew and photographed many German and Italian military installations during the months preceding World War II.

The main military user of the Lockheed 12 was the Royal Netherlands East Indies Army Air Force, which bought 36.
Sixteen of these were the Model 212, a version created by Lockheed for training bomber crews, which had a .303 in (7.696 mm) caliber machine gun in an unpowered, partly retractable gun turret on top of the fuselage, a second .303-caliber machine gun fixed in the nose, and bomb racks under the wing center section that could hold eight 100 lb (45 kg) bombs.
The other 20 aircraft were transport versions.

Several Lockheed 12s were used as technology testbeds.
The U.S. National Advisory Committee for Aeronautics (NACA) bought two, adding a center vertical fin to each of them to test stability improvements.
One of the NACA Lockheed 12s was used to test "hot-wing" deicing technology, in which hot exhaust air from the engines was ducted through the wing's leading edge to prevent ice accumulation.

Three other Lockheed 12s were used to test tricycle landing gear.
These had their normal landing gear replaced by a non-retracting version with a large nosewheel and with the main wheels shifted further back on the engine nacelles. (
The tailwheel from the normal conventional gear was retained.)
The gear was non-retractable because there wasn't room within the structure to stow it in retracted position.
Streamlined fairings were placed on the gear to reduce drag.
One of the tricycle gear Lockheed 12s went to the U.S. Navy as the XJO-3 and performed carrier landing tests on the USS Lexington to study the suitability of a twin-engined tricycle-gear aircraft for carrier operations.
Another went to the U.S. Army as the C-40B, and still another was retained by Lockheed for its own testing; both of these were eventually converted back to the normal landing gear configuration.

Milo Burcham flew a Lockheed 12A in the 1937 Bendix Trophy Race from Burbank, California to Cleveland, Ohio.
This 12A had extra fuel tanks in the cabin, allowing it to save time by making the entire 2,043 mi (3,288 km) trip non-stop.
The 12A came in fifth at an average speed of 184 mph (296 km/h); this was an impressive performance, since the first and fourth-place winners were both privately owned Seversky P-35 fighters.

Another Lockheed 12A, owned by Republic Oil Company and named The Texan, was modified by aviator Jimmie Mattern for a round-the-world flight attempt.
Mattern filled the 12A's cabin with fuel tanks and removed the cabin windows and door; the crew would enter the aircraft via a cockpit hatch.
The aircraft was denied a U.S. permit for the flight following the Earhart incident; however, it was pressed into action in September 1937 in a long range search effort for Sigizmund Levanevsky, who crashed somewhere
Even though the Lockheed 12 had won the government's feeder airliner competition, most of the airlines rejected it, and very few Lockheed 12s were used as airliners. One notable airline user was the newly renamed Continental Air Lines, which had a fleet of three Lockheed 12s that ran on its route between Denver, Colorado and El Paso, Texas in the late 1930s. Another was British West Indian Airways Ltd., which flew the Lockheed 12 on Caribbean routes in the Lesser Antilles during the mid-1940s.
The Lockheed 12 proved much more popular as a transport for company executives or government officials. Oil and steel companies were among the major users. A number were purchased as military staff transports by the United States Army Air Corps, which designated the type as the C-40, and by the United States Navy, which used the designation JO, or in one peculiar case, R3O-2. With the arrival of World War II, many civilian Lockheed 12s were requisitioned by the U.S. Army and Navy, Britain's Royal Air Force, and the Royal Canadian Air Force.
Two civil Lockheed 12s ordered by British Airways Ltd. were actually intended for covert military reconnaissance flights. Sidney Cotton modified these aircraft for aerial photography and in civilian guise, overflew and photographed many German and Italian military installations during the months preceding World War II.
The main military user of the Lockheed 12 was the Royal Netherlands East Indies Army Air Force, which bought 36. Sixteen of these were the Model 212, a version created by Lockheed for training bomber crews, which had a .303 in (7.696 mm) caliber machine gun in an unpowered, partly retractable gun turret on top of the fuselage, a second .303-caliber machine gun fixed in the nose, and bomb racks under the wing center section that could hold eight 100 lb (45 kg) bombs. The other 20 aircraft were transport versions.
Several Lockheed 12s were used as technology testbeds. The U.S. National Advisory Committee for Aeronautics (NACA) bought two, adding a center vertical fin to each of them to test stability improvements. One of the NACA Lockheed 12s was used to test "hot-wing" deicing technology, in which hot exhaust air from the engines was ducted through the wing's leading edge to prevent ice accumulation.
Three other Lockheed 12s were used to test tricycle landing gear. These had their normal landing gear replaced by a non-retracting version with a large nosewheel and with the main wheels shifted further back on the engine nacelles. (The tailwheel from the normal conventional gear was retained.) The gear was non-retractable because there wasn't room within the structure to stow it in retracted position. Streamlined fairings were placed on the gear to reduce drag. One of the tricycle gear Lockheed 12s went to the U.S. Navy as the XJO-3 and performed carrier landing tests on the USS Lexington to study the suitability of a twin-engined tricycle-gear aircraft for carrier operations. Another went to the U.S. Army as the C-40B, and still another was retained by Lockheed for its own testing; both of these were eventually converted back to the normal landing gear configuration.
Milo Burcham flew a Lockheed 12A in the 1937 Bendix Trophy Race from Burbank, California to Cleveland, Ohio. This 12A had extra fuel tanks in the cabin, allowing it to save time by making the entire 2,043 mi (3,288 km) trip non-stop. The 12A came in fifth at an average speed of 184 mph (296 km/h); this was an impressive performance, since the first and fourth-place winners were both privately owned Seversky P-35 fighters.
Another Lockheed 12A, owned by Republic Oil Company and named The Texan, was modified by aviator Jimmie Mattern for a round-the-world flight attempt. Mattern filled the 12A's cabin with fuel tanks and removed the cabin windows and door; the crew would enter the aircraft via a cockpit hatch. The aircraft was denied a U.S. permit for the flight following the Earhart incident; however, it was pressed into action in September 1937 in a long range search effort for Sigizmund Levanevsky, who crashed somewhere
Lockheed Model 12 Electra Junior