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', -header => [
                        buttongroup => [buttons => [
                            ['Click me', ['#'], data => { holder => 'claw'}],
                            ['Tap-tap', ['#'], caret, items => [
                                ['Me too', ['#']]
                            ]],
                        ]]] => begin %>
    <p>A short text.</p>
%  end

<div class="panel panel-default">
    <div class="panel-heading">
        <h3 class="panel-title">
            <div class="btn-group pull-right">
                <a class="btn btn-default" data-holder="claw" href="#">Click me</a>
                <div class="btn-group">
                    <a type="button" href="#" data-toggle="dropdown" class="btn btn-default dropdown-toggle">Tap-tap <span class="caret"></span></a>
                    <ul class="dropdown-menu">
                        <li><a tabindex="-1" href="#" class="menuitem">Me too</a></li>
                    </ul>
                </div>
            </div>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>