1941 Plus-X Negative film (5231) initially introduced under the name Plus-X Panchromatic Negative film (4231).
1959 Double-X Negative film (5222) introduced.
The two films have remained comparitively unchanged since. There have been improvements throughout the long production run of these two emulsions. In the late 90's there was a change in the emulsion hardener. In the early 2000's both emulsions were re-engineered and production moved into a state-of-the-art coating facility at Kodak. This resulted in greater improvements in the film's overall uniformity and batch consistency.
Between 2002 and 2005 there were several improvements in the components of the emulsions, overcoats and gel layers. In 2007 significant improvements in the support layers of Double-X and Plus-X were made using more modern antistatic technology.