Generate XML from oracle11 DB

I need to generate xml format given below from an oracle 11 database. All tag names are static strings. Data for nodes D1, D2, D3 etc I am able to get in a single query using a 2 table join.

If I use XMLElement("Node1", XMLElement("Node2",XMLElement("Node3",XMLElement("D1", ..... then even Node1, Node2 is getting returned with each row of data. I need Node1 and Node2 only once open at top and close at bottom. Any idea how can I do that?

<Node1>
   <Node2 key = "XYZ">
        <Node3>
            <D1> R1F1 </D1>
            <D2> R1F2 </D2>
            <D3> R1F3 </D3>
        </Node3>
        <Node3>
            <D1> R2F1 </D1>
            <D2> R2F2 </D2>
            <D3> R2F3 </D3>
        </Node3>
        <Node3>
        <D1> R3F1 </D1>
        <D2> R3F2 </D2>
        <D3> R3F3 </D3>
        </Node3>
   </Node2>
</Node1>