E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
When Washington was in want of ammunition, he called a council of officers, but no practical suggestion could be offered. We must consult brother Jonathan, said the general, meaning his excellency, Jonathan Trumbull, the elder governor of the State of Connecticut. This was done, and the difficulty was remedied. To consult brother Jonathan then became a set phrase, and brother Jonathan grew to be the John Bull of the United States. (J. R. Bartlett: Dictionary of Americanisms.)