"an American girl"
Not all feminine words end with ة (tá marbútah). There are many words in Arabic that are feminine without this letter to their ends; Example: شمس (šams: sun), أرض (arDH: Earth).
This said, the main purpose for ة is actually to combine the sounds of (H) and (T) into a single letter for grammatical reasons. Because these words ending with (H) sound, would change their endings to (T) when they undergo specific changes and vowels are added, or even suffixes are added, to their ends.
However, words like بنت here, has the (T) sound originally in the word. Meaning, if I say this word alone, without any effects, or at the end of the sentence (where vowels are usually dropped at the last letter of the last word), the spelling remains the same: Bint. The (T) sound here does NOT change to (H) under any circumstances.