Betty Boop: "There's Something About A Soldier" (1934) 

From Wikipedia: Betty is recruiting soldiers for a war against mosquitoes. She offers a kiss to anyone who enrolls, which grabs the interest of several men. When Fearless Fred joins, he is stripped to his underwear, revealing his true fat, which is eventually pressed into muscle weight and looks like a real soldier. The war on the mosquitos parodies World War I-style combat. When the townspeople win, they all celebrate, and Betty and Fred kiss.