You should clarify what you mean by distributed transaction.
If the transaction use different JCA connections inside the server the only thing is to have all such resources handle XA (two phase commits) correct.
Also a client can initiate the Tx if it is using the same server (you need transaction stickyness in this case).
If you mean that the Tx span different server instances, i.e. two EJB's on different servers you need to have a JTS configuration instead of the (default) JTA. There are scripts in the docs directory to change the configuration.