Paul Tchistopolskii has created an XML utility to generate DTD from an XML document. As an example, submit Bank.xml

 <?xml version="1.0"?>
 <Bank>
  <Number>1234</Number>
 </Bank>

and the DTD generator returns the DTD

 <?xml version="1.0" encoding="UTF-8"?>
 <!ELEMENT Bank (Number)>
 <!ELEMENT Number (#PCDATA)>