This is a realistic feature of the aircraft which is caused by it's unique aerodynamics. You will need to adjust the trim after take off to keep it level, you can also help this by varying the amount of power you have set (less power = less nose up pitch).

If you are unsure how to set the trim, here is a FSX keyboard command list that you can view and download by clicking on the 'download' button below. You can find the trim commands under 'Control Surface':