No, letters of intent are not required. Submission of a letter of intent is not binding and the letter is not part of the application review.
Sometimes a letter of intent is requested by the funding opportunity announcement (FOA) as a way for NIH staff to begin planning for the review of your application. We encourage you to submit the letter of intent. However, NIH welcomes your application even if you chose not to submit the letter.
When requested, letters of intent are due 30 days before the application due date. The FOA will specify what information should be included. Generally the letter of intent includes the FOA number, a descriptive title of the proposed activity, the name of the applicant institution, the name, address and phone number of the PI(s), and identifies other key personnel and participating institutions.