Does placing a limit order at the bid price essentially bypass the bid/ask spread? Or does it just wait until ask price falls to your limit?

I.e. If I buy a stock that has a $99 bid price and a $100 ask price, if I bought it at the market price I'd pay $100, with the brokerage pocketing the $1 difference.

If I placed a limit order for $99 though, would the order execute immediately? Or would that require that the ask price falls to $99?