Citroën actually introduced the first production unibody in 1935 - Ford in 1936, and Nash in 1941. 1949 Chrysler Imperial was the first car standard with Disc B

Citroën actually introduced the first production unibody in 1935 - Ford in 1936, and Nash in 1941. 1949 Chrysler Imperial was the first car standard with Disc Brakes, Mercedes can however take credit for the crumple zone, and probably side-impact reinforcement. Unibody as a safety feature is more complex, Unibody cars are not inherently safer (quite arguably a perimeter frame may be safer for some collisions), and until recently they were often heavier for the same size car - it wasn't until the late 70's-80's where a unibody could be lighter than a BOF car because of enhanced computer modeling - generally a unibody however is safer than the X-frame as exemplified in this video.