Updated by Howard Aero as the Howard 250, the conversion was to prove popular with about 22 aircraft modified in the early 1960s in San Antonio Texas.

 

• Some of the upgrades include:
• Increased fuel capacity to 1100 gallons
• Gross takeoff weight increased to 22,500 lbs.
• Extended wing tips
• Specially developed square tip propellers
• Engine cowling to nacelle streamlining
• High flow exhaust system
• Two speed superchargers
• Engine fire extinguishers
• Electric elevator trim
• In-flight fuel dump system
• Streamlined flap and wing trailing edges
• Fully enclosed landing gear doors
• 12 place oxygen system
• Stretched cabin picture windows
• Curved glass windshields
• Extended cockpit side windows
• Autopilot
• Extended nose for Bendix weather radar
• Freon air conditioning
• Stinger tail for streamlining
• Deice boots

 

Lockheed began to develop a larger and updated version of the Model 14 Super Electra in 1939 after the earlier design did not prove a commercial success.

 

The company modified the fourth Super Electra by adding two more rows of seats in a fuselage stretched by 5ft 6in to 49ft 10in. The first of three altered aircraft flew on September 21, 1939, with the first true Model 18 following on February 2, 1940.