What are all the proper nouns in this sentence On the morning of August 6 1945 an American bomber released an atomic bomb over Hiroshima a city of 34300 people in Japan?

A proper noun is the name or title of a specific person, place, or thing.There are two proper nouns in the sentence:

• August, the name of a specific month;
• Hiroshima, the name of a specific city.

Note: The Word 'American' in this sentence is functioning as a proper adjective, describing the noun 'bomber'. The noun 'American' is a Word for a person.