The class is set to .panel-default, by default:

%= panel

<div class="panel panel-default">
    <div class="panel-body">
    </div>
</div>

---

If you want a panel without title, set the title to undef:

%= panel undef ,=> begin
    <p>A short text.</p>
%  end

<div class="panel panel-default">
    <div class="panel-body">
        <p>A short text.</p>
    </div>
</div>

---

%= panel 'The Header' => begin
    <p>A short text.</p>
%  end

<div class="panel panel-default">
    <div class="panel-heading">
        <h3 class="panel-title">The Header</h3>
    </div>
    <div class="panel-body">
        <p>A short text.</p>
    </div>
</div>

---

A success panel with a header:

%= panel 'Panel 5', success, begin
    <p>A short text.</p>
%  end

<div class="panel panel-success">
    <div class="panel-heading">
        <h3 class="panel-title">Panel 5</h3>
    </div>
    <div class="panel-body">
        <p>A short text.</p>
    </div>
</div>