Ol' Ben did make the proposal. But some historians who are familiar with his wirtings, think he actually wrote it in jest, to make fun of the selection of the bald eagle which is a bad tempered bird, with tendencies for thievery and eating carrion.
Don't believe me, about the eagle? Go to Alaska. Bald eagles are attractive looking, but watching them fight with ravens over a half rotted salmon rack and a cheeseburger wrapper or eat mushed pollock guts out of a mud puddle in Dutch kinda dimishes their stature a bit.