Take a Letter, Darling (1942)

Tom Verney is dubious about his new duties as "secretary" to advertiser A.M. 'Mac' MacGregor, which include escorting her socially to confound amorous clients and jealous wives. After an abortive pass, Tom is almost convinced that Mac means (only) business. But attitudes change when Tom is pursued by pretty Ethel Caldwell and Mac by her rich brother Jonathan.