Do I need a letter of invitation or Affidavit of Support to receive a visitor visa from the United States?

Do I need a letter of invitation or Affidavit of Support to receive a visitor visa from the United States?

No, you do not. 

According to the U.S. State Department...

Visa applicants must qualify on the basis of the applicant's residence and ties abroad, rather than assurances from U.S. family and friends. A letter of invitation or Affidavit of Support is not needed to apply for a visitor visa. If you choose to bring a letter of invitation or Affidavit of Support to your interview, please remember it is not one of the factors used in determining whether to issue or deny the visa.