We know according to this article(wrote by the historian Jonathan Steinberg) in the british magazine The Economist we know he was the roommate of an American at the university of Göttingen and they were friend

John Motley, an American who was Bismarck's roommate at Göttingen University, saw the will to dominate in his friend when he was only 18 years old. In an 1839 novel Motley thinly disguised him as Otto von Rabenmarck who tells the narrator, a new student at Göttingen: “I intend to lead my companions here, as I intend to lead them in after-life.”

In the year 1832, when, at the age of seventeen, he went as a student to Goettingen, he spoke English fairly well. He apparently wished to have more practice; at least we hear that he celebrated the fourth of July with an American party the same year that he entered.

And according to the historian Kenneth Weisbrod in his book The Atlantic century Bismarck was pro American

After the Civil War ended, Bismarck sounded out Washington on an alliance.

When Sidney Whitman, the famous English writer, paid a visit to Bismarck after the latter had retired from office as Chancellor to his estate Friedrichsruh, near Hamburg, he was much astonished to find in his study the pictures of Ulysses Grant, Walter Phelps, and George Bancroft, but the picture of only one Englishman—Lord Beaconsfield.

“I gleaned,” writes Whitman in his recollections of Bismarck, “that his personal relations with the Americans whom he had met in the course of his career, had invariably been of a most cordial character. Nearly all of them were attracted to him by the charm of his manner.”

In 1890 a memorial celebration was held at Goettingen in honor of four prominent Americans who had studied there. Bismarck was invited to attend. In his telegram declining the invitation, he said: “Of the four distinguished Americans to be honored by memorial tablets, I have the privilege of counting two among my intimate friends: the late John L. Motley and George Bancroft.”

In summary: We know that Bismarck had American friends and was attract by the American culture , and we know he tried to seek an alliance with the USA after the Civil war

Sources:

The Atlantic century, Kenneth Weisbrod

Bismarck: A Life. By Jonathan Steinberg. Oxford University Press