According to Wikipedia, the first Leica prototypes were built by Oskar Barnack at Ernst Leitz Optische Werke, Wetzlar, in 1913. Intended as a compact camera for landscape photography, particularly during mountain trips, the Leica was the first practical 35 mm camera that used standard cinema 35 mm film. The Leica transports the film horizontally, extending the frame size to 24×36 mm, with a 2:3 aspect ratio, instead of the 18×24 mm of cinema cameras which transport the film vertically.
The Leica went through several iterations, and in 1923 Barnack convinced his boss, Ernst Leitz II, to make a pre-production series of 31 cameras for the factory and outside photographers to test. Though the prototypes received a mixed reception, Ernst Leitz decided in 1924 to produce the camera. It was an immediate success when introduced at the 1925 Leipzig Spring Fair as the Leica I (for Leitz camera). The focal plane shutter had a range from 1/20 to 1/500 second, in addition to a Z for Zeit (time) position.
Below are images showing main timelines of Leica cameras' history:
(via Street Chronicles)