Problem with XmlHttpRequest Javascript

this script logs 'null' for the xml object, I know I should use jQuery, but I would like to use an XHR object on this occasion, since I am working with another company's API as well and it would be really nice to know how to use XHR with quickbase

function procVehicleXml(){
    var xml;
    var request = new XMLHttpRequest();
    request.open("POST", rprUrl, true);
    request.onreadystatechange = function(){
        if (this.readyState === 4 && this.status === 200){
            xml = this.responseXML;
            console.dirxml(xml);
        }
    };
    var apiMethod = {
        apptoken: token,
        action: "API_DoQuery",
        clist: "35"
    };
    request.send(JSON.stringify(apiMethod));

}