Originally released just one month after the bombing of Pearl Harbor, to encourage children to protect themselves from chemical attacks and to do it in god damn style. According to Major Robert D. Walk, a former Weapons of Mass Destruction Individual and Instructor Training Officer at the US Army Reserve Command:

The mask was designed so children would carry it and wear it as part of a game. This would reduce the fear associated with wearing a gas mask and hopefully, improve their wear time and, hence, survivability.

The Mickey Mouse Gas Mask was produced as part of the war production program. The Sun Rubber Company produced approximately 1,000 Mickey Mouse gas masks and earned an Army-Navy ‘E’ for excellence in wartime production in 1944. Overall, production of the Noncombatant Gas Masks (and in fact, all gas masks) was one of the most successful production programs of the war. In fact, production had to be curtailed early due to the vast quantity produced.

Very few of the Mickey Mouse gas masks survived. The US Army Chemical Museum at Fort McClellan, Alabama, has a hand-made prototype. The 45th Infantry Division Museum, Oklahoma City, Oklahoma, has a production specimen on permanent display with other gas masks in the combat support area of the museum. The Walt Disney Archives, Burbank, California, has a facepiece without ears, lenses, or a canister, and a mask owned by the founder of the Sun Rubber Company was on display at the Summit County (Ohio) Historical Society’s "Toys Made in Summit County" exhibit in 1982.

Thanks for the info, Major Walk. But it's still pretty creepy.