<a class="thumbnail" target="_blank|_self|_parent|_top|framename">

1. 为表单添加属性target="_blank"

<form method="get" id="form" target="_blank">
    <div class="input-group" style="margin-top: 150px;margin-left: 260px;">
        <span class="input-group-btn">
            <button id="baidu" class="btn btn-primary" type="submit">百度一下</button>
        </span>
        <input id="search" type="text" class="form-control" placeholder="Search for..." name="wd">
    </div><!-- /input-group -->
</form>