Horizontal submenu (dropdown) in Bootstrap 3

Question

I am trying to get the submenu in Bootstrap's navbar to open horizontally, instead of vertically.

I hoped this would be something as simple as adding an extra style, e.g.

ul.dropdown-menu li {
    display:inline;
}

or

.navbar-nav li.dropdown li {
  display:inline;
}

but nothing changes.

I couldn't find anything useful after quite a bit of searching either. Hope to get some help here.