How to format responses of ec2 endpoint to json - rest

Question

I am trying to perform a GET-Request to the ec2-Endpoint https://ec2.eu-central-1.amazonaws.com/?Action=DescribeInstances&Version=2016-11-15 in order to get a list of all ec2 instances within my aws account (in eu-central-1 region). Unfortunately the response's content is formatted in xml. Is there a chance to change the format of the response to json?
I've already set the GET-Request's Accept-Header to "application/json" but without success.
If I query the iam-endpoint https://iam.amazonaws.com/?Action=ListUsers&Version=2010-05-08 with the same Accept-Header the response's content is being delivered properly in json string format.
These are the headers for the ec2-query (not working):
GET /?Action=DescribeInstances&Version=2016-11-15 HTTP/1.1
Host: ec2.eu-central-1.amazonaws.com
Accept: application/json
Content-Type: application/x-www-form-urlencoded
X-Amz-Date: 20180706T091958Z
Authorization: <valid AWS4 authorization header>
Cache-Control: no-cache
Postman-Token: 69c8f349-95b5-4e9d-991c-5ce2a55b0cbe
And the headers of my iam-query (working):
GET /?Action=ListUsers&Version=2010-05-08 HTTP/1.1
Host: iam.amazonaws.com
Accept: application/json
Content-Type: application/x-www-form-urlencoded
X-Amz-Date: 20180706T092222Z
Authorization: <valid AWS4 authorization header>
Cache-Control: no-cache
Postman-Token: ad86f2ae-870c-4289-a9d3-1f2ad9082c8f
Executing aws cli tools command aws ec2 describe-instances lists all instances in json format, so I think there should be a way to achieve this with a GET-request.
Thank you very much for your help.

Answer 1

Probably too late, but might help others.
I also tried to get json output from DescribeInstances without luck.
It seems to be impossible at the moment, since even aws cli receives it in xml and then converts it to json. You can see it using --debug flag:
aws ec2 describe-instances --debug