American WW2 M1941 Field Jacket.
Officially called "Jacket, Field, OD", it was introduced in May 1941 and was worn throughout World War Two. The M41 jacket was designed before the war and was to replace the 4 pocket wool service tunic for field use.
After field test's before the war by General Parsons this was the design that was decided best suited combat field use by the US Army.