This is a quite old question but without an accepted answer. The answer you are looking for might be surprisingly simple: concatMap.

When a promise is created, it starts execution immediately, so they are executing in parallel; while when values are emitted from one observable, they are in serial.

So combine these two, for the following code snippet, observables from promise are executed in parallel, and the result of them are emitted in serial because concatMap puts them into one stream in the order they are created.

Rx.Observable.from(urls_array)

.concatMap(function(url) { return Rx.Observable.fromPromise(Promise.resolve($.get(url))) })

.subscribe(

  function(jsonObj) {

    // first result will arrive first

  },

  function(err) { },

  function() {

    // all completed

  }

)