Actually, on 1918 grounds, I would dispute that the bomber would always get through; early RFC and RAF Independent Force efforts were hellaciously expensive, on their bad days as costly and ineffective as anything 2 Group managed in 1940. In the end the night bombers, relatively safe but inaccurate, were spending as much as sixty percent of their time (heavies less, mediums more) on trying to bomb airfields and flak batteries to give the day bombers some chance of getting through the defences. Going the other way, the Gothas and Reisen were more effective, landing a few painful hits, but were facing increasing defences, transitioned almost entirely to night bombing by september 1917, and even then were whittled down to ineffectiveness by may- june 1918, by which point they were pulled back to tactical targets anyway.
Early strategic bombing ended in defensive victory, shock value notwithstanding, and every air force in the world tried to persuade themselves that it would be different next time. Claiming that the bomber would always get through was a combination of (im)pious hope and justification of budget and role, not the product of reasoned evidence- based analysis.
Based on 1917-18, the unescorted bomber is running a Red Queen's Race between anything it can achieve and the costs it imposes on it's own side, waste and loss through accident and misadventure, and even semi- organised defences can easily tip the balance against them; 1940 was actually lessons learnt on both sides- escorted bombers against well organised defence. The defence still won, as long as they could see to fight.
Sticking to rifle calibre machine guns delays the dominance of the defence a fair bit, actually; but the Zeppelin-Staaken Reisen series were probably the last bombers that could really expect to win a gun duel with a defending fighter. Again, actual analysis from actual results rather than basing strategy and force structure on a service politics based wish list should have shown this. The other lesson of 1917-18 is that attacking the defences directly is sometimes necessary, and can and does pay off in reducing your own losses.
Which way the game of attrition breaks- whether the damage lost by the diversion of effort to defence suppression is more or less than the damage prevented by the enemy defences- depends a lot on the weapons and conditions of the campaign. Defence suppression including escorting fighters, of course, and also arguably including bomber armour and turrets. Historically, there is an argument that a higher flying, faster bomber almost entirely stripped of direct defence (might be worth leaving a nominal capability just to ward off cheap kills), relying on an accompanying suppression platform like a fighter, is better off than an armed and armoured bomber. If you can get a fighter with the range, which 1918 often couldn't.
Also, most of the fighters of the late Great War had an overload capability- they could carry bombs. The Camel and SE.5a had two hundred pound loads, eight 20 or 25lbr. Your route to an escort fighter may be through not forgetting this fact- remember that it could be done, and came in damn' handy on occasion, a fighter being able to attack and survive in the face of ground fire that would have plucked a bomber out of the sky. Keep building fighters with that in mind. Do not allow a fighter specification to go out without it- don't build light bombers at all. Enemy fighters cut them to ribbons over the trenches, regularly. Fighters that can jettison and fight back if necessary are a much better bet.
And when the need for a long range fighter comes up, add plumbing to the bomb, now also drop tank, racks. Problem solved. The Spitfire and Hurricane mark I should have been capable of it.