Does the ADA require employers to develop written job descriptions?

No. The ADA does not require employers to develop or maintain job descriptions. However, a written job description that is prepared before advertising or interviewing applicants for a job will be considered as evidence along with other relevant factors. If an employer uses job descriptions, they should be reviewed to make sure they accurately reflect the actual functions of a job. A job description will be most helpful if it focuses on the results or outcome of a job function, not solely on the way it customarily is performed. A reasonable accommodation may enable a person with a disability to accomplish a job function in a manner that is different from the way an employee who does not have a disability may accomplish the same function.

For additional information, take a look at the following resources:

FAQ: Does the ADA require that an applicant or employee with a disability be qualified for the position?

FAQ: What are the  “essential functions” of a job?