A 50% reduction in fuel consumption is reasonable, even if the 2020 timeframe sounds very ambitious:
Let's start out with the engines:
A geared fan in combination with a recuperator and an intercooler could reduce SFC by about 15%, improvements in other parts (compressor, burner and turbine) could give another 5%.
So lets say the engines can give an SFC that is reduced by 25%.
Have a look at what needs the engine. The engines are needed by the airframe itself. Why? Because the airframe produces drag. In a first-order approximation, drag is weight divided by glide number. So for the airframe to increase efficiency, we need to a) reduce weight and b) reduce drag (or increase lift with a given drag).
a) How do we reduce weight? Well weight is mass times gravitational acceleration. Modifying the former might be a possibility, but not in 2020; so we're stuck with reducing the mass: Use lighter materials and most of all, optimise the structure. "Better" carbon fibres are possible, stronger alloys, whatever. a 5% reduction in OEM sounds possible for the given timeframe.
b) Reducing drag: Here comes the difficult thing. There won't be any quantum leaps in aerodynamic efficiency (Cl over Cd) with our ancient Stable-B47-Configuration. Sure, modified wingtips can be one thing. Non-smooth surfaces to reduce drag are another (they might easily give like 5% however); but we have to ditch the configuration. What options have we? 1) Keep the configuration and go unstable. That could give you about 5% basically for free. 2) Move the ailerons to the front (canard configuration). Gives you a rough 5% as well. 3) Use a lifting body (be it BWB, lifting body only or all-wing-configuration) Could give you somewhat more than 5%, but possibly not that much. However, all in all, an improvement of aerodynamic efficiency of 12% is a possibly reasonable assumption.
c) Other technologies & systems. Mainly improve interaction between components. One thing that really jumps to my mind is BLI
(boundary layer ingestion), others are more electrical aircraft and stuff. Lets say this area give us another 3% in improvement of aerodynamic efficiency and 2% reduction in OEM.
Please note that the absolute numbers here do not matter in the following analysis.
Now, lets assume we have a sample aircraft with a Zero-Fuel Mass of 1 and a Takeoff Mass of 1.6 (i.e. 37.5% of the mass is fuel on takeoff), and aerodynamic efficiency of 1, a flight velocity of 1, and specific fuel consumption of 1, and lets set the gravitational acceleration to 1 for simplicity. This gives us an overall, first order range of 0.47. This is our baseline aircraft.
Now, we have a new aircraft with a Zero-Fuel Mass of 0.93, a (to be computed Takeoff Mass), and aerodynamic efficiency of 1.15, a flight velocity of 1 and a specific fuel consumption of 0.75, again the gravitational acceleration of 1. And we want a range of 0.47 as well. This gives us a Takeoff Mass of 1.26.
The baseline aircraft needed 0.6 units of fuel for the given mission.
The new aircraft needed 0.33 units of fuel for the given mission, or a reduction of 44%.
This was achieved by reducing SFC by 25%, aerodynamic efficiency improved by 15% and Empty Mass per Seat reduced by 7%.