MEFO Bills are a German 民族精神 introduced with the 
Waking the Tiger DLC.
MEFO bills were promissory notes created by Hjalmar Schacht to enable the government to fund rearmament, acting through the balance sheet entity Metallurgische Forschungsgesellschaft to hide this rearmament from French and British eyes. These bills must be extended for six-month periods, or their recipients will have to be paid when they fall due. This national spirit is designed to emulate the German rearmament in the period of the Four Year Plan leading up to the war in Europe. As such, its main focus is buffing military related construction.
MEFO Bills mechanic
The national spirit is automatically extended every 180 days unless a decision to not continue is taken. MEFO bills require initially the expenditure of +0.20
/day, which after each extension increases by another +0.15/day. This increase in cost can be counteracted with some historical national focuses, namely Anschluss, End of Czechslovakia and Fate of Yugoslavia, all of which lower the daily cost by -0.10/day.
If Germany starts - or gets embroiled in - a war, the MEFO Bills spirit gets suspended, its buffs and costs are no longer applied. After the war's conclusion, it gets replaced by the national spirit Mefo Bills due, which gives -130 and +20%
for 170 days.
The same effect takes effect if the decision to stop the Mefo Bills national spirit is taken.
MEFO Bills effects
Consumer Goods Factories: -5%- Daily 政治点数 Cost: +0.20
Military factory construction speed: +25.00%
空军基地 construction speed: +25.00%
海军基地 construction speed: +25.00%
Naval dockyard construction speed: +25.00%
陆上要塞 construction speed: +25.00%
海岸要塞 construction speed: +25.00%
Anti Air construction speed: +25.00%
合成炼油厂 construction speed: +15.00%
Fuel Silo construction speed: +25.00%
雷达站 construction speed: +25.00%
MEFO bills strategy
MEFO bills serve to steer the buildup phase of a historical Germany towards an aggressive expansion of military production and infrastructure. The German focus tree, by virtue of the Four Year Plan branch and the Anschluss branch, allows for expansion of civilian industry without compromising on military production. However, the German buildup is also flexible enough to follow a different approach.
There are two main approaches to the matter: either using the construction buffs to the fullest to maximize military output or concentrate on a civilian buildup early on and only use the industrial might of Germany in 1938/39.
The merits of the former approach are that Germany needs to build an air force and maybe a submarine or surface navy from scratch, which requires a prodigious amount of time if meaningful numbers are to achieved. Additionaly, the +25% speed bonus in construction is the most powerful in the early years, before construction technology and advisor buffs can add their weight. Furthermore, as the Anschluss branch national focuses are locked behind manpower in the field requirements, equipment must be produced early on. Military factories adding vital equipment and producing enough of it to keep the army and air force training for XP generation and trained divisions/wings helps to further increase the military power of the Wehrmacht.
The downside of this is that future german industrial expansion then heavily relies on expansion and occupation and the buildup is curbed by the need to use civilian factories for trade, as much of the additional production would require 
, 
or 
. Under wartime embargoes, the lack of raw materials might become critical.
The latter approach, building up a civilian economy and only later increasing military production, avoids the pitfalls of early military expansion. The national focuses of the Four Year Plan Branch and the very affordable advisor Hjalmar Schacht make this a viable strategy for Germany. It requires to postpone the Anschluss branch focuses, which however gives the nations annexed by these focuses more time to increase their industrial potential and produce more equipment to be seized. The major risk of this approach is that if the player wants to start the war in late summer 1939, the German military power may be too small to quickly win the first battles. This can result in critical lack of equipment, as building military factories becomes harder with MEFO bills gone after the war begins.