The prototype for the revised airliner, designated Model 18 by Lockheed, was converted from the fourth Model 14, one of a batch which had been returned to the manufacturer by Northwest Airlines after a series of crashes.
Prototipe untuk pesawat direvisi, dirancang menjadi Lockheed Model 18, konversi dari Model 14 keempat, salah satu dari batch yang telah dikembalikan ke produsen oleh Northwest Airlines setelah serangkaian insiden.