As others mentioned, the Higgins boats were designed to put as many troops on the beach as possible. A flat bottomed boat, the Higgins could pull into very shallow water and drop the ramp quite literally on the sand.

Also, keep in mind that military doctrine of the era was far more in favor of such tactics (and to an extent, it still is): The quickest way to overwhelm an enemy defensive line that you can only take with a full frontal assault is to put as many troops into the battle as quickly as possible. Because the enemy cannot take out all of your troops indefinitely, you will have troops that will get around the fixed fortifications and take them out from behind and eventually open up gaps for more to pour through, thus causing the enemy's defensive line to collapse as a whole.

For what it's worth, military doctrine and technology has changed a lot in the past 70 years. Lessons learned in the Pacific and on D-Day showed the vulnerability of troops in open-air transports and the dangers of front ramps in assaults with the first wave.

Amphibious assault troops on a first wave would go in on AAVs which are armored amphibious assault vehicles like this one. Note the rear ramp

Also, mobility has changed a lot. Rather than directly assault a fortified beach, we have more options than ever to go around them. Helicopters add a new dimension as now you can fly above or around fortified beachheads. LCACs (the hovercraft you hear about) add speed to land heavy weapons on beaches the enemy wouldn't expect. Of course, that's not to say that the US doesn't use craft similar to that on D-Day still: the Landing Craft, Mechanized still very much is used but its primary purpose is no longer to assault defended beachheads.

I think it's important to remember that the D-Day landings were not intended to occur without the support of armor. When the troops hit the beaches on foot it was expected that there would be tanks there to support them. And that, I think, changes the equation dramatically. Because then you have your own heavy guns and your own machine guns keeping the machine guns and so on from the beach fortifications in check.

And, indeed, that happened at several of the landings. But at Omaha few tanks made it ashore and most of those were quickly knocked out of action so the troops had to assault the beach fortifications on foot.

More so, because those landings were so much more dramatic and astounding than the landings that went more smoothly they tend to get the majority of the attention historically. But it's not as though it was ever intended for the assault of Omaha beach to proceed as it did. But in war things often go wrong and more often than not it's necessary merely to persevere and try to make the best of it, which, of course, they did.

I think the big assumption here is that there is some kind of a better design out there. If you have troops disembark from the rear of the craft they might be in water over their heads. Trying to getting a bunch of men to jump into the cold atlantic fully loaded isn't going to be easy. Lowering a ramp won't be much of a help if the water is deep. Plus everything about that operation is going to be slow as troops have to disembark and turn around to let the next soldier disembark. This makes the rear ramp craft a sitting duck for shore artillery. Even worse, the troops are just going to have to move past the bow of the ship to get to shore anyway, or move parallel to the beach which is going to also increase the exposure time to enemy fire. It doesn't really matter where you put the exit door, there just ins't a better way to do it. No matter how you slice it, landing troops under fire is going to be costly.

It is also just a movie. It makes a very powerful scene, but doesn't necessarily describe reality. I am not saying that no one was ever cut down as soon as the ramp dropped, but it clearly didn't happen to everyone.

You have some decent answers, but there's a lot of missing information as well with regards to boat design and tactics. Fouled beaches were a big concern and design was driven as much by the need to get the boat OFF the beach as much as getting the personnel, vehicles, and supplies TO the beach. The military needed a common design that could work with both vehicles and personnel - the common landing craft you see was a LCVP, which stood for Landing Craft, Vehicle Personnel (which was different from the LCPR, "Landing Craft Personnel Ramp). In order to get both types off quickly, you need to get them as close to the beach as possible. Because a beach is sloped, if you drop them off the back, you drop them into deeper water, and if you do that with a "conventional" vehicle (most of the jeeps and others had some light waterproofing) you're more likely to swamp it so that it can't move, and then you can't back the boat out and can foul the landing area. Boat swamping was also a series issue, as some of the [documentation shows](http://www.researcheratlarge.com/Misc/SurfTraining/10L-39SurfTraining.html] - the jury-rigged surf plate in the linked document is part of the design of the later LCPR / LCVP (you can see it as the angled shape immediately aft of the "gunner's cockpit" in this drawing. Opening the rear of the boat to incoming surf would be a monstrously bad idea.

Additionally, the rear end of a boat grounded at the bow can be very "dynamic" what with the surf pushing it around. Consider that the propeller is in this same area; any solders who lose their footing in the surf are in just as much danger of being crushed, drowned, or ripped open if they exit the rear.

Finally - the boats were not designed to protect the soldiers and sailors at all. The ramps were metal for strength, but not armor, and the rest of the boat was wood. Machine guns bullets would pass through the ramp like a hot knife through butter unless it was at extreme range.

A good book on the subject of the boats during the war is Andrew Jackson Higgins and the Boats That Won World War II by Jerry E. Strahan

Another question...during the invasion, why did our air force (army air corps at the time I beleive) and navy not completely obliterate the German forces protecting the beaches before sending thousands of soldiers to be bullet sponges?

I think it is also important to remember, in addition to what has already been mentioned, that the boats were intended to quickly leave the beach and turn around to go pick up more troops and carry them ashore (and in later assault waves, perhaps bring wounded back). As such, having the door at the front allows the soldiers to quickly get out while allowing the boat to reverse and turn around. While they might have been rather simple in their construction compared to other military equipment, they were still valuable and you wouldn't them sitting in place on the beach too long to take fire. Soldiers exiting from the back and getting stuck for whatever reason would provide a challenge for the infiltration of the boats. Soldiers might get wounded back there (there was indirect fire to worry about beyond just machine guns), or they might get stuck by obstructions on the seafloor, or they may simply seize up out of fear and not want to leave the safety of the cover behind the landing craft.