A gas turbine engine has a first spool having a low pressure compressor section in fluid communication with an air inlet, the low pressure compressor section including a first plurality of variable guide vanes therein, and a low pressure turbine section drivingly engaged to the low pressure compressor section. A second spool has a high pressure compressor section in fluid communication with the low pressure compressor section to receive pressurized air therefrom, the high pressure compressor section including a second plurality of variable guide vanes at an entry thereof, and a high pressure turbine section drivingly engaged to the high pressure compressor section, the high pressure turbine section disposed upstream of the low pressure turbine section and in fluid communication therewith. An output drive shaft drivingly engages the low pressure turbine section and is adapted to drivingly engage a rotatable load of the gas turbine engine.